Eable entra ID authentication

[DOliana]

Motivation and Context

Currently access to Azure OpenAI requires a key to be used. I wanted to enable entra ID based authentication. There is an issue for this: #190

Description

• make the setting of AZURE_OPENAI_KEY optional
• add description to README on how to configure RBAC

Contribution Checklist

• [x] I have built and tested the code locally and in a deployed app
• [x] This is a change for all users of this app. No code or asset is specific to my use case or my organization.
• [x] I didn't break any existing functionality :smile: