Support blob signing with external account credentials where possible.

See SO post: https://stackoverflow.com/questions/76266019/how-do-i-generate-signed-urls-for-gcs-with-workload-identity-in-the-c-sharp-sdk/76266912#76266912

Currently workload id is not supported for generating signed URLs for GCS. This is a problem because it forces a bad practice (generating a static json key instead of using workload id which is much more secure).

At the very least can someone provide a full example of how to use the SignBlob method in conjunction with SignedURL to work around this problem?

Also, I think default app creds are also not supported right? This makes the code painful to run locally for testing purposes.