feat: add AzureConfigKey::CredentialType to select Azure credential method (#710)

When multiple Azure credential configurations are present in
environment variables (e.g. both workload identity and client secret),
the builder's fixed resolution order may pick an undesired credential.
This adds a `credential_type` config key that lets users explicitly
select which credential method to use, bypassing the priority chain.

Supported values: auto, bearer_token, access_key, client_secret,
workload_identity, sas_token, azure_cli, managed_identity.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Co-authored-by: Andrew Lamb <andrew@nerdnetworks.org>

Author: 3 people