Add `MailDeliveryJob` for delivering both regular and parameterized mail.
Deprecate using `DeliveryJob` and `Parameterized::DeliveryJob`.

Deliver parameterized mail with `ActionMailer::DeliveryJob`
and remove `ActionMailer::Parameterized::DeliveryJob`.

- If a Mail defines a custom delivery_job, all ActionMailer assertion
  helper (assert_emails, assert_enqueued_emails ...) wouldn't work.

  ```ruby
    MyMailer < ApplicationMailer
      self.delivery_job = MyJob
    end

    # This assertion will fail
    assert_emails(1) do
      MyMailer.my_mail.deliver_later
    end

  This PR leverage the new ActiveJob feature that accepts Procs for the
  `only` keyword and check if the delivery job is one of ActionMailer
   registered ones.

Setting parameterized_delivery_job on a mailer class will cause Parameterized::MessageDelivery to use
the specified job instead of ActionMailer::Parameterized::DeliveryJob:

    class MyMailer < ApplicationMailer
      self.parameterized_delivery_job = MyCustomDeliveryJob
      ...
    end

Since these changes related to the public API, I think we should add
changelog entries.

Related to #33838, #33849

Example of `assert_enqueued_with` with no block
```ruby
def test_assert_enqueued_with
  MyJob.perform_later(1,2,3)
  assert_enqueued_with(job: MyJob, args: [1,2,3], queue: 'low')

  MyJob.set(wait_until: Date.tomorrow.noon).perform_later
  assert_enqueued_with(job: MyJob, at: Date.tomorrow.noon)
end
```

Example of `assert_enqueued_email_with` with no block:
```ruby
def test_email
  ContactMailer.welcome.deliver_later
  assert_enqueued_email_with ContactMailer, :welcome
end

def test_email_with_arguments
  ContactMailer.welcome("Hello", "Goodbye").deliver_later
  assert_enqueued_email_with ContactMailer, :welcome, args: ["Hello", "Goodbye"]
end
```

Related to #33243

* ActionMailer::Base can unregister observer(s) and interceptor(s).

One or multiple mail observers can be unregistered using
`ActionMailer::Base.unregister_observers` or
`ActionMailer::Base.unregister_observer`.

One or multiple mail interceptors can be unregistered using
`ActionMailer::Base.unregister_interceptors` or
`ActionMailer::Base.unregister_interceptor`.

For preview interceptors, it's possible to use
`ActionMailer::Base.unregister_preview_interceptors` or
`ActionMailer::Base.unregister_preview_interceptor`.

* Ensure to be reset registered observer(s) and interceptor(s)

* Add explanation to CHANGELOG

* Add original author's name

[Kota Miyake + Rafael Mendonça França + Claudio Ortolina]

* Eager autoload mail gem when eager load is true

We had a production issue where our Sidekiq worker threads all became
deadlocked while autoloading a file within the mail gem, required via
ActionMailer, despite setting our Rails applicaiton to eager load.
`Mail.eager_autoload!` exists and works great, ActionMailer just doesn't
call it during eager loading. Adding it to the ActionMailer Railtie's
eager_load_namespaces takes care of calling `Mail.eager_autoload!`
during the `eager_load!` initializer.

* 'Mail' isn't defined yet, use before_eager_load instead

* Make sure mail is loaded

* Move eager load of Mail into ActionMailer.eager_load!

[Samuel Cochran + Rafael Mendonça França]

Skipping over 2.4.0 to sidestep the `"symbol_from_string".to_sym.dup` bug.

References #32028

:tada::tada::tada:

* Add missing credit
* Add backticks
* Fix indentation
* Remove trailing spaces

And some minor tweaks.

PR #29270 changed the number of arguments that gets passed to Procs
defined in ActionMail::Base.default. With this changeset, Procs can
now have 1 or 0 arguments

Also adds test coverage for AM::Base.default Proc arity.

Setting delivery_job on a mailer class will cause MessageDelivery to use
the specified job instead of ActionMailer::DeliveryJob:

    class MyMailer < ApplicationMailer
      self.delivery_job = MyCustomDeliveryJob

      ...
    end

[ci skip]

Offer the option to use parameterization for shared processing of headers and ivars

Follows the same pattern as controllers and jobs. Exceptions raised in
delivery jobs (enqueued by `#deliver_later`) are also delegated to the
mailer's rescue_from handlers, so you can handle the DeserializationError
raised by delivery jobs:

```ruby
class MyMailer < ApplicationMailer
  rescue_from ActiveJob::DeserializationError do
    …
  end
```

ActiveSupport::Rescuable polish:
* Add the `rescue_with_handler` class method so exceptions may be
  handled at the class level without requiring an instance.
* Rationalize `exception.cause` handling. If no handler matches the
  exception, fall back to the handler that matches its cause.
* Handle exceptions raised elsewhere. Pass `object: …` to execute
  the `rescue_from` handler (e.g. a method call or a block to
  instance_exec) against a different object. Defaults to `self`.

- Fixed statement about setting `config.action_mailer.default_url_options = {protocol: 'https'}` . We are just setting the protocol key to 'https', not replacing/initializing the complete config.
- Fixed grammar in assert_emails changlog
- Added sentence separator for code ":"

[ci skip]

They would be lost when the delivery job is enqueued, otherwise.
Prevents a common, hard-to-find bug like:

```ruby
message = Notifier.welcome(user, foo)
message.message_id = my_generated_message_id
message.deliver_later
```

The message_id is silently lost here! *Only the mailer arguments are
passed to the delivery job.*

This raises an exception now.

Make modifications to the message within the mailer method or use a
custom Active Job to manage delivery instead of using #deliver_later.

Removes `-t`

Adds changelog headers for beta3 release

Whenever you are sending emails in integration tests using the `:test`
delivery method you need to make sure that
`ActionMailer::Base.deliveries` is reset after every test. This piece of
boilerplate code is present in all my applications that send
emails. Let's have `ActionDispatch::IntegrationTest` reset the
deliveries automatically.

[ci skip]

During the `5.0.0.beta1` release, the CHANGELOGs got an entry like the
following:

```
* No changes.
```

It is kinda confusing as there are indeed changes after it. Not a
biggie, just a small pass over the CHANGELOGs.

[ci skip]