support NewUserConn operation

* feat: include RunId in FRP Server Plugin NewProxy message

* doc: rewrite server plugin documentation

* refactoring monitor code, support prometheus
* remove vendor

token is the current token comparison, and oidc generates oidc token using client-credentials flow. in addition - add ping verification using the same method

This variable didn't seem to be used anyway, so no further changes were
required.

The ServerCommonConf is now passed around as an argument instead of
being shared between components as a global variable. This allows for
more natural interaction with server.Session as a library and allows for
multiple servers to co-exist within the same process.

Related: #1387

The client and server services now only attempt to load assets if the
dashboard or admin panel are enabled. This change makes it possible to
use FRP as a library without having to manage assets. If a library user
wants to start a server with the dashboard enabled, they will need to
set the DashboardPort and AssetsDir fields of ServerCommonConf.

Just nitpicky typo fixes.