Synching favorites and Playlist with Subsonic

[frigi83]

The usage of the music provider Subsonic is at the moment one way. On the otherway the provider Spotify has a synchronization in both ways.

The idea is to sync favorites and Playlist in both ways. A change in MA will be visible on Subsonic and the otherway.

Others app that uses the Subsonic API, are able to do the synchronization. I think that should be possible to do that also in MA. But I am not a developer and I don't have any idea were to start.

A solution like this allow to have the same Playlist in my home and on the way with the smartphone (done time offline).

[khers]

This PR begins to address your request: music-assistant/server#1335

[frigi83]

Thank's @khers! I have tested the version 2.1.0 BETA 3.

Works when adding a new provider, so I think that the issue #2359 can be closed.

I hope that synchronization in both directions, remains as the ultimate goal.

[khers]

The last piece of synchronization is favorites marked in MA sent back to the subsonic server. I will be working on that next.