Fix `Style/StringLiterals` violations for Active Storage

```
% be rubocop -a --only Style/StringLiterals activestorage
Inspecting 74 files
........................................CCCCCCCCCC.C........CC.......C.C..

(snip)

74 files inspected, 31 offenses detected, 31 offenses corrected
```

Use File::NULL instead of "/dev/null"

`Relation::Merger` should not fill `values` with empty values

Fix `find_by` with range conditions

Unlock minitest for Rails' test suite

Refactoring `Response#charset=`

Sync transaction state when accessing primary key

Add Azure to the available services list [ci skip]

Add Azure to readme [ci skip]

previously https://github.com/rails/activestorage/pull/86

Add Azure configuration to Active Storage template

Use duktape gem as default JS engine on Windows-MINGW

With Active Storage supporting Azure, we should show people how to set it up.

Incorporates the minor changes originally in https://github.com/rails/activestorage/pull/88 as well

The fallback javascript engine on Windows is Windows Script Host (JScript).
However this engine isn't able to process the default assets, because it supports
ES3 only but the coffeescript compiler requires ES5.

Fixes #30014

Since this test changed in 92209356 I noticed that it really doesn't make
sense anymore. I split the tests into 2 groups to explain what each one
does.

First these routes should throw a `bad_request` when the encoding isn't
valid. We're expecting UTF8 encoding and passing binary, that should be
a bad request.

For the second test we are setting the `show` route to set
`self.binary_params_for?` for that route which will convert the
parameters and return a `:ok` instead of a `:bad_request`.

Remove unnecessary `doc` directory deletion

Let ActiveStorage tests pass only for Disk

Remove HTTParty dependency

"httparty" is only added in #30020 to write two tests to make PUT requests
against S3 and GCS.

The same requests can be made with net/http, removing a dependency from the Gemfile.