fix: Subsonic: Add test for fetching Open Subsonic Extensions (#171)

This lets users verify if their server is fully compatible and I should have included it from the beginning.

docs/music-providers/subsonic.md

@@ -16,6 +16,7 @@ There is also a toggle for podcasts which will have no effect now but will allow
 ## Known Issues / Notes
 - Not all server implementations accept an empty string as a search query, however this is considered valid input per the API documentation. If search or track enumeration fails, ask the authors of your server implementation about handling empty query strings.
 - This provider makes use of https://github.com/khers/py-opensonic for communicating with the server, if something is failing to work properly in Music Assistant, try to use that library to interact with your server (can you ping it?, fetch artist and albums?, can you search?).
+- This provider only supports servers implementing the Open Subsonic API definition. To verify that your server is compatible, use the same setup you used to test connectivity above to hit the getOpenSubsonicExtensions() endpoint. If this endpoint is not implemented, we cannot talk to your server.
 - If you find a mismatch between what is displayed by your Subsonic compatible server and Music Assistant then refer to and contribute [here to help find a solution](https://github.com/music-assistant/support/issues/2192)
 - Not all Open Subsonic implementations handle tracks/albums with multiple contributing artists particularly well. If you see strange artists listed in Music Assistant, please verify that your implementation has an artist ID for all artists listed on a track or album. See the discussion [here](https://github.com/music-assistant/hass-music-assistant/issues/2965).
 - If problems are encountered seeking within tracks try the "Force player provider seek" option in the subsonic provider settings