DefaultAzureCredential supports public cloud only

[chlowell]

The default credential is currently a chain of EnvironmentCredential, ManagedIdentityCredential, and (on Windows) SharedTokenCacheCredential.

These credentials unevenly support multiple clouds:

• EnvironmentCredential accepts an authority kwarg but this is undocumented because the credential is intended to be configured entirely by environment variables. Selecting cloud via environment variable requires a cross-language design and implementation in azure-core (Azure.Core & Azure.Identity needs to be updated to support env vars #8094).
• cloud selection is meaningless for ManagedIdentityCredential (and any fix must consider that)
• SharedTokenCredential accepts an authority kwarg (for identity, this is what changes between clouds)

I think the best solution has two parts:

• azure-core implements cloud selection by environment variable
• DefaultAzureCredential accepts an authority kwarg, and passes it only to credentials expecting it. We can pass it to EnvironmentCredential as a workaround pending azure-core changes.