Add support to enable observers to sent paintings to one player and its allies

[Garanas]

Description

Extends the functionality of https://github.com/FAForever/fa/pull/6726.

Add a lobby option to allow all observers to send brush strokes to a specific player and all of its allies. This is useful for trainers coaching their pupil(s) during a live session. Instead of just talking over voice, the trainer can use the painting feature to convey additional meaning.

Course of Action

This would require extending the adapters to add support for a single player to listen for brush strokes sent by observers. This should depend on a lobby option. The lobby option would identify the army id that listen for brush strokes from observers. The recipient and all of its allies can then receive and view these brush strokes.

There are no changes required for the observer to view the brush strokes of players. This is already the case.

The game would be unrated in this situation.