improvement: handle azure workload identity authentication

[abestel]

Title:

• Handle azure workload identity authentication

Description: (optional) So far the Azure OpenAI integration was handling authentication using Client ID / Client Secret and Managed identity using the IMDS endpoint which is deprecated in favor of Workload Identity (using the public OAuth2 endpoint of Entra ID).

This changeset aims at handling this new authentication type.

Note that this requires reading environment variables set by the Azure runtime onto the virtual machine / pod using a workload identity. It also needs to read a file on disk (containing an assertion to use to exchange against a JWT).

Motivation: (optional)

• managed identity authentication using IMDS is deprecated and should be replaced by Workload Identity authentication

Related Issues: (optional) /