feat (provider/azure): add support for Azure Managed Identity authentication #3124

[ShervK]

Fixes: #3124

Added a parameter to pass a bearer token provider with the @azure/identity package to createAzure. This way, its similar to the Azure documentation on how to use managed identity as well as how the OpenAI node package handles Azure Managed Identity.

Example:

import { createAzure } from '@ai-sdk/azure';
import { streamText } from 'ai';
import {
  getBearerTokenProvider,
  DefaultAzureCredential, // Can use any type of Credential module, not just Default
} from '@azure/identity';


const credential = new DefaultAzureCredential();
const scope = 'https://cognitiveservices.azure.com/.default';
const azureADTokenProvider = getBearerTokenProvider(credential, scope);

async function main() {
  const provider = createAzure({
    resourceName: process.env.AZURE_RESOURCE_NAME,
    identityTokenProvider: azureADTokenProvider,
  });
 
   ...
}

Implemented by creating a fetch wrapper and injecting the Authorization token into the headers on each request, similar to how the OpenAI package grabs a new token for each request. I tried implement it in the getHeaders function but ran into issues with the async operation of the token provider, so I went with the fetch wrapper instead since I've had to use this method for managed identity.

I can add a section on how to setup and use managed identity to the azure provider docs if that would help. I also added some tests for the token provider, let me know if this would need more or less tests.