Consolidate backends and cleanup dependencies

[nrfulton]

We have a lot of dependencies, and the exact dependency groups required for a given backend or feature are sometimes surprising. We should minimize dependencies in the mellea core.

Agreed upon changes:

• remove outlines

Proposals:

• Consolidate on a few backends in mellea itself -- probably just openai and litellm and move other backends into mellea-extras-backends or similar. Use dependency groups there like we use them here.