Add extras and hubs mixins

[JonnyWong16]

Description

Refactor extras() to an ExtrasMixin and hubs() to HubsMixin.

• Adds ExtrasMixin to Artist, Track, Movie, Show, Season, and Episode objects.
• Adds HubsMixin to Artist, Movie, Show, and Collection objects.

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)
• 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 (Tests already exist for extras and hubs)