new feature: support custom credential providers in Azdls

[zakariya-s]

Feature Description

Add credential_providers and builder methods to AzdlsBuilder, similar to the existing custom credential-provider APIs for S3 and GCS.

Problem and Solution

Azdls currently constructs its credential chain from configured static credentials and reqsign’s default Azure providers. Applications that obtain short-lived Azure credentials externally cannot inject a provider into a long-lived Azdls operator. The custom provider state should be stored on AzdlsBuilder and installed in the reqsign signer when the backend is built.

This allows reqsign to consult the provider again when a cached credential becomes invalid, without rebuilding the operator or retrying storage operations.

Additional Context

No response

Are you willing to contribute to the development of this feature?

• Yes, I am willing to contribute to the development of this feature.