Add ability to provide a custom credential in AzureAISearchVectorStore

[joerattazzi-microsoft]

Description

My team is using GraphRAG's AzureAISearchVectorStore class. Where we're using this, we need the ability to provide a credential directly, as we can't set api_key and DefaultAzureCredential doesn't meet our needs.

By default, AzureAISearchVectorStore class will:

• check for an api_key to be set
• if no api_key is provided, default to DefaultAzureCredential

Our proposed change will add the support in this way:

• check for an api_key to be set
• if no api_key is provided, use a provided credential if one is given.
• if no other input is provided, default to DefaultAzureCredential

Related Issues

#1695

Proposed Changes

Our proposed change will add the support in this way:

• check for an api_key to be set
• if no api_key is provided, use a provided credential if one is given.
• if no other input is provided, default to DefaultAzureCredential

Checklist

• I have tested these changes locally.
• I have reviewed the code changes.
• I have updated the documentation (if necessary).
• I have added appropriate unit tests (if applicable).

Additional Notes

This probably won't solve use cases through the API, but it should enable future expansion if needed