Support Databricks foundational models APIs

[zacdav-db]

• [x] DBRX
• [x] Meta Llama 3 70B
• [x] Mixtral 8x7B

Databricks endpoints are OpenAI compatible and have therefore only needed to make minor changes. path in configs is set to be /serving-endpoints/ (docs) because the request URL depends on workspace (DATABRICKS_HOST).

[zacdav-db]

Outstanding items:

• [x] Resolve behaviour with chattr_app()
• [x] Tests passing
• [x] Add user agent to identify {chattr}?
• [x] Vignette

[zacdav-db]

@edgararuiz I believe that should be g2g if you can please review.

[edgararuiz]

Everything looks great! Would you mind adding a NEWS item? Once done, I'll merge. Thanks!

[edgararuiz]

Thanks @zacdav-db !!