Support Azure AI Serach Service keyless authentication

[asaikali]

The current AzureVectorStoreAutoConfiguration expects an api key, however there are many organization that don't allow api key access and leverage the azure managed system identity.

todo:

• enhance the AzureVectorStoreAutoConfiguration so that it can use system identity
• enhance docs to explain to users how this can be done

related to #1624

Resources:

• https://github.com/Azure-Samples/azure-openai-keyless-java

[RodrigoDiasDeOliveira]

To resolve this issue, you can update the AzureVectorStoreAutoConfiguration to allow authentication via Azure Managed Identity instead of requiring an API key. This method is more secure and practical for organizations that prefer to avoid API keys.
i holp it helps!

[mattgotteiner]

I think this already works pretty well. I'm working on addressing @markpollack's feedback so this is more usable

#1683

[markpollack]

@mkheck @TheovanKraay Is this something that you can help out with please?

related to #2707