Storage: signer

We need to created signed URLs. Some general comments:

- We should separate this from the client used for uploads and downloads, the configuration have almost zero overlap
- We should support only V4 signed URLs
- We should support signed policy docs
- We need additional features in the `GoogleCloudAuth` library to sign blobs
- Not all credential types can sign blobs, we prefer compile-time checks vs. runtime checks to deal with this (e.g. no function to create a "signer" vs. a signer that fails at runtime)

And some small details:

- [ ] Write the `storage_generate_signed_post_policy_v4` sample
- [ ] Write the `storage_generate_signed_url_v4` sample
- [ ] Write the `storage_generate_upload_signed_url_v4` sample

Start by reviewing the GoogleCloudAuth library and the requirements for separating signing from upload and download clients, supporting only V4 signed URLs and signed policy documents, and handling credential capabilities at compile time. The work is complete when the signer features are implemented and the storage_generate_signed_post_policy_v4, storage_generate_signed_url_v4, and storage_generate_upload_signed_url_v4 samples are written.

Written by the indexing model from the issue text.