Add ability to toggle account view state syncing

[JonnyWong16]

Description

• Adds new MyPlexAccount.viewStateSync property to determine if view state syncing is enabled/disabled for the account.
• Adds new methods MyPlexAccount.enableViewStateSync() and MyPlexAccount.disableViewStateSync() to enable and disable view state syncing for the account, respectively.

Ref: https://forums.plex.tv/t/sync-watch-state-and-ratings-announcement-feedback/800885

Type of change

Please delete options that are not relevant.

• [x] New feature (non-breaking change which adds functionality)
• [x] This change requires a documentation update

Checklist:

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