[Feature Request]: Accept Credential in AzureAISearchVectorStore

[joerattazzi-microsoft]

Do you need to file an issue?

• [x] I have searched the existing issues and this feature is not already filed.
• [x] My model is hosted on OpenAI or Azure. If not, please look at the "model providers" issue and don't file a new one here.
• [x] I believe this is a legitimate feature request, not just a question. If this is a question, please use the Discussions area.

Is your feature request related to a problem? Please describe.

AzureAISearchVectorStore assumes that you're either able to provide an api_key or that the DefaultAzureCredential will work as a fallback.

When operating in a system where DefaultAzureCredential won't work as needed (such as via Azure Synapse), api_key isn't always an option due to security concerns.

Under the covers, AzureAISearchVectorStore uses these to create a Credential to use. I'm proposing an option to provide a credential directly when leveraging AzureAISearchVectorStore as a new input.

Describe the solution you'd like

A new kwargs entry is added for credential such that it can optionally be provided. It would be added in a non-breaking manner such that the order of preference for a Credential to use internally does not break active users:

1. the api_key
2. the provided credential input (new)
3. Fall back to the DefaultAzureCredential

Additional context

No response