Support Plex resuming and playback offset

[jjlawren]

Proposed change

Support two new parameters in the media_content_id payload for Plex media_player.play_media service calls:

• resume: Resume playback from the last known position
• offset: Begin playback at X seconds from the beginning

If both are included, offset will take precedence.

Examples:

service: media_player.play_media
target:
  entity_id: media_player.plex_player
data:
  media_content_type: movie
  media_content_id: '{ "library_name": "Movies", "movie.title": "Elf", "resume": true }'

service: media_player.play_media
target:
  entity_id: media_player.plex_player
data:
  media_content_type: tvshow
  media_content_id: >-
    {
      "library_name": "TV Shows",
      "show.title": "Futurama",
      "season.index": 5,
      "episode.index": 2,
      "offset": 300            # Begins playback 5 minutes into the episode
    }

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 Plex play_media doesn't support current_time parameter #58982
• This PR is related to issue:
• Link to documentation pull request: TODO

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

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[bdraco]

This looks fine. The service taking a json string seems a bit odd though... I don't know the history here, but was there a reason something like this can't work?

service: media_player.play_media
target:
  entity_id: media_player.plex_player
data:
  media_content_type: tvshow
  media_content_id:
      library_name: "TV Shows"
      show.title: Futurama
      season.index: 5
      episode.index: 2
      offset: 300           

[jjlawren]

Legacy conventions. Could possibly be refactored to accept both ways but it is probably better to come up with a new convention to handle this more cleanly in all media players.