feat: New OpenAI compat embeddings API

[hardikjshah]

What does this PR do?

Adds a new endpoint that is compatible with OpenAI for embeddings api. /openai/v1/embeddings Added providers for OpenAI, LiteLLM and SentenceTransformer.

Test Plan

LLAMA_STACK_CONFIG=http://localhost:8321 pytest -sv tests/integration/inference/test_openai_embeddings.py --embedding-model all-MiniLM-L6-v2,text-embedding-3-small,gemini/text-embedding-004