Support for Artist radio stations

[jjlawren]

Description

This adds support for artist radio stations. In Plex Web, this is represented by the "Play Radio" button on an artist page.

An artist radio station can be retrieved using Artist.station(), which returns a Playlist if it exists. To differentiate these radio stations from classic playlists, a new radio boolean attribute is exposed.

PlayQueue creation has been updated to handle these special radio Playlist instances, which allows PlexClient.playMedia to accept them as-is.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated the docstring for new or existing methods
• I have added tests when applicable