Fix Agent object settings

[JonnyWong16]

Description

• Fix mediaType argument for PlexServer.agents() not converted to the searchType ID.
• Fix parsing Agent settings enumValues.
• Make Agent.settings() method public and add DeprecationWarning to _settings()

Fixes #863

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

[ReenigneArcher]

I have confirmed that the enum values are returned with your changes. Thanks for addressing this so quickly! Hopefully no issues with merging it.