Add local signing to cloud storage signed urls

[ahyerman]

What component of google-cloud-cpp is this feature request for? GCS (i.e., something in google/cloud/storage)

Is your feature request related to a problem? Please describe. GCS CLI supports local signing where a private key can be passed as part of command but client libraries do not support this (from what I can tell, this is true for all libraries- java, c++, etc.)

gsutil: https://cloud.google.com/storage/docs/gsutil/commands/signurl#usage
gcloud: https://cloud.google.com/sdk/gcloud/reference/storage/sign-url

Describe the solution you'd like Support for local key usage and signing

Describe alternatives you've considered The only alternative now is to use CLI or write my own signing script.

[dbolduc]

Yeah, I think the client library only supports providing a service account as the google::cloud::storage::SigningAccount, e.g.:

https://github.com/googleapis/google-cloud-cpp/blob/main/google/cloud/storage/examples/storage_signed_url_v2_samples.cc

https://github.com/googleapis/google-cloud-cpp/blob/main/google/cloud/storage/examples/storage_signed_url_v4_samples.cc