Add Plex library count sensors

[jjlawren]

Proposed change

Adds a new sensor for each Plex library showing the total number of items it contains. Also shows count attributes for special cases, such as shows & seasons for TV and artist & albums for music.

Associates all sensors with the existing Plex server device and gives that device a more appropriate name.

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 #
• This PR is related to issue:
• Link to documentation pull request: Describe new Plex library sensors home-assistant.io#17145

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.

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

• Documentation added/updated for www.home-assistant.io

The integration reached or maintains the following Integration Quality Scale:

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

[bdraco]

LGTM. Two small comments above

[MartinHjelmare]

We don't set unavailable state directly. We modify the return value of the available property.

[jjlawren]

Good catch, I'll fix. 👍