Add local signing to cloud storage signed urls

**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.