Improve Plex device handling

[jjlawren]

Breaking change

Devices representing Plex Web media_player devices will be removed. Any automations, scenes, or scripts based on the device will need to be changed to use the media_player entity.

Proposed change

This PR removes orphaned devices where the user has removed the associated media_player entity.

The method for assigning devices for certain Plex media_player entities is also changing. Plex Web and Plex for Sonos media_player entities represent potentially transient apps or external services. For example, a new Plex Web device is created for each browser login. These should not have permanent devices attached to them. This PR migrates the entities to a new shared "Plex Clients" service. This will also allow cleanup of the old individual devices during a subsequent restart.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes Device registry does not clean itself when no entity linked to it #42317
• This PR is related to issue:
• Link to documentation pull request:

Checklist

• The code change is tested and works locally.
• Local tests pass. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the development checklist
• The code has been formatted using Black (black --fast homeassistant tests)
• Tests have been added to verify that the new code works.

The integration reached or maintains the following Integration Quality Scale:

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To help with the load of incoming pull requests:

• I have reviewed two other open pull requests in this repository.

[balloob]

Shouldn't we just return None?

[jjlawren]

I originally wanted to delete the current devices, but that also removed the associated entities. This moves them to a new service device instead so the old device can get safely removed by the new orphan cleanup code.

[jjlawren]

BTW, I think I'd actually prefer to not have any devices attached to these entities, but returning None doesn't disassociate them from existing devices.