Ragna has a few logos builtin

This is neither scalable nor compatible with the fact that users can specify their own assistants. We should have an Assistant.avatar() -> str (name TBD) method or property that defaults to the same behavior that we have right now. This method or property should be honored by the UI.

Tasks:

1. Define Assistant.avatar() -> str as method or property on the base class.

2. Implement 1. on all builtin assistants

3. Figure out a way to make the avatar available through the engine. IMO, it would be best if we could somehow fit it into

   ragna/ragna/deploy/_engine.py

   Line 95 in 00ffa2b

   def get_components(self) -> schemas.Components:

   but maybe a standalone function is simpler for now.

4. Use the returned avatar from the engine in the UI

5. If we chose to use a standalone function in 3., add a new endpoint to the API.