Standardize Plex server connections

[jjlawren]

Breaking Change:

Logins using a username/password/servername are no longer supported on the Plex sensor platform. This functionality will be partially restored in followup PRs which add a configuration flow and external authentication.

Description:

First step in using common code between the Plex media_player and sensor platforms. This adds a common class to connect to Plex servers.

This is a smaller part of #26157.

Pull request with documentation for home-assistant.io (if applicable): home-assistant/home-assistant.io#10297

Example entry for configuration.yaml (if applicable):

sensor:
  - platform: plex
    host: 192.168.1.5
    token: abc1234xyz

media_player:
  - platform: plex

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

If the code communicates with devices, web services, or third-party tools:

• Untested files have been added to .coveragerc.

[MartinHjelmare]

Great!

[dshokouhi]

Should we also consider changing the configuration to being included under the plex: domain? I know might be out of scope of this PR but I figured this integration should become like the others where it discovers the media player and sensor after accessing the server.

[MartinHjelmare]

Yeah, we're talking about that. But it's not for this PR.