Move config and connections to Plex component

[jjlawren]

Breaking Change:

Configuration is moved from the media_player and sensor platforms to the plex component. Existing configurations will need to be updated. The remove_unavailable_clients and client_remove_interval options for media_player entities have been removed as the functionality has limited value now that the entity registry exists.

The plex component will now enable both sensor and media_player platforms by default, sharing the same credentials.

Old example:

media_player:
  - platform: plex
    host: 192.168.1.5
    token: mysecrettoken
    use_episode_art: False

sensor:
  - platform: plex
    host: 192.168.1.5
    token: myothersecrettoken

New example:

plex:
  host: 192.168.1.5
  token: mysecrettoken
  media_player:
    use_episode_art: False

Description:

A continuation from #26157 and #26458.

Configuration for the media_player and sensor plex platforms have been merged into a shared configuration. Connection logic has been centralized and both platforms now share the same connection object. The legacy configurator configuration method has been preserved and now works for both platforms instead of just media_player. Discovery has also been modified to load the plex component instead of the Plex media_player component.

Pull request with documentation for home-assistant.io (if applicable):
home-assistant/home-assistant.io#10328

Example entry for configuration.yaml (if applicable):

See Breaking Change above.

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the development checklist

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in home-assistant.io

[MartinHjelmare]

Great!