To support generating signed URL for GCS, we need to implement support for calling IAM signing endpoints via some supported Credential types in the Rust SDK.

Some ideas:

• Have a given Credentials builder provide a Signer trait (via build_signer() method) that can later be used on a bucket.signed_url(object_name, signer) method in GCS.

https://docs.cloud.google.com/storage/docs/access-control/signed-urls