Remove default host for Plex config

[jjlawren]

Breaking Changes:

• Removes default value of host config parameter.
• Requires one of host or token in the configuration.

Description:

Preparation for #26548 to avoid multiple HA releases with breaking changes in Plex.

Previous PRs for this release include #26444, #26458, #26488

Adds support for configuration by token and optionally server. If configured this way, will retrieve connection URL from Plex service. The server config option is only used if multiple Plex servers are attached to the account associated with the provided token.

Pull request with documentation for home-assistant.io (if applicable): home-assistant/home-assistant.io#10342

Example entry for configuration.yaml (if applicable):

plex:
  host: 192.168.1.10
  port: 32400
  token: MYSECRETTOKEN

or

plex:
  token: MYSECRETTOKEN
  server: 'My Plex Server Name'   # Optional if multiple servers tied to Plex account

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]

Can be merged when build passes.