Change the default value for the authority host option to be read from the environment variable first.

[ahsonkhan]

Fixes #4875

Ultimately, this isn't a breaking change, but rather a bug fix to match user intention:

1. If the user set the option before to a particular value, it continues to get honored (since that is the highest precedence)
2. If the user doesn't set the option, and was running in an environment where the environment variable wasn't set, we previously fell back to the default authority (which is the public cloud), and we continue to do so, since the env var value will be empty.
3. If the user doesn't set the option, and was running in an environment where the environment variable was set, we now correctly use that value over the fallback default authority. That's the bug fix here.

If, for some reason, the user really wanted the previous behavior, and ignore the environment variable set on the environment, they can explicitly set the AuthorityHost option to the desired value. This way the intent is explicit and clear, rather than implicit/accidental.

[joshfree]

Added Discuss in Office Hours

[antkmsft]

Can you please cover the "read-from-the-environment" behavior with tests?
See CredentialTestHelper::EnvironmentOverride.

And if you permanently keep "" = public cloud, then you probably also need some tests via CredentialTestHelper::SimulateTokenRequest, that the request URL is the authority URL.
Otherwise, I think, just creating the options and making sure the AuthorityHost matches the env variable, is sufficient (each credential / common credential impl already has tests verifying that the URL of the request being made does match the authority URL supplied via options).

[antkmsft]

I am approving with an assumption that #5002 will be addressed before the release and the "get option value from the environment" tests will be written. I don't mind if that comes in subsequent PRs. If that is not what you're planning to do, I'd like to talk more.

[antkmsft]

I think you're unintentionally introducing a regression in WorkloadIdentityCredential (see comment for workload_identity_credential.cpp below)