Allow adding items from different servers to playlist

[JonnyWong16]

Description

Allow adding items from different servers to playlist.

Playlist.addItems(items) automatically splits the items into separate API calls since Plex only allows adding items from one server at a time. This will keep the items in the order they added in the list.

Closes #1409

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