add support for AZURE_TOKEN_CREDENTIALS

[cataggar]

Feature Summary

The AZURE_TOKEN_CREDENTIALS can be used to customize DefaultAzureCredential.

Feature Description

The Azure Identity libraries added support for an AZURE_TOKEN_CREDENTIALS environment variable to DefaultAzureCredential. This feature allows developers to choose between Deployed service or Developer tool credentials by setting the environment variable to prod or dev, respectively.
https://devblogs.microsoft.com/azure-sdk/azure-sdk-release-june-2025/

It allows specifying a specific credential, just like

• add azure_identity::create_credential(), SpecificAzureCredential, AppServiceManagedIdentityCredential, VirtualMachineManagedIdentityCredential  #1532
  But the env var is AZURE_TOKEN_CREDENTIALS and not AZURE_CREDENTIAL_KIND. It also supports categories like prod and dev.

Nice to see this adopted.

Use Case

No response

Alternatives

No response

Additional Context

No response

[chlowell]

Thanks for opening this! In the last release we replaced DefaultAzureCredential with the much simpler DeveloperToolsCredential (#2873). This new type is a chain of just two credentials designed for local development scenarios. We don't want to add a credential selection feature for it because we want to keep it simple and encourage using specific credential types instead of chains, and it only has two credentials to select anyway.

All that said, we do have a code example demonstrating how to implement this behavior with a custom credential type: specific_credential.rs

[cataggar]

Please reconsider this in the future. I would like to see this added back to match all other SDKs out of the box.
https://devblogs.microsoft.com/azure-sdk/azure-sdk-release-june-2025/#azure-identity-libraries:-defaultazurecredential-customization-improvement