Collection: take provider as input parameter

[nishika26]

Is your feature request related to a problem? Please describe.
Collections are currently hardcoded to use OpenAI for vector store operations. As we move towards supporting multiple LLM providers, we need the ability to specify which provider a collection should use.

Describe the solution you'd like
Add a provider parameter to Collection creation:

Implementation Details:

• Add provider: str field to Collection model
• Default value: "openai" (maintains backward compatibility)
• Validate provider value against supported providers