fix: Make auth tokens resolved dynamically per request.

[mgiannakopoulos]

Summary

WIth this change we:

• Dynamically resolve authentication and obo tokens per LLM/Embedding request.
• We allow passing through or resolving via env variables the hosting_platform and the endpoint_url of the provided InferenceAPI model.

JIRA Issues

MCDB-86327 MCDB-86334