go-cloud based uris

[maraino]

Description

Add support to go-cloud style URIs compatible with cosign

[hslatman]

This seems to have been completed for the most part with #240. At least how they're used and processed.

@maraino I noticed they use gcpkms instead of cloudkms. Same is true for e.g. azurekeyvault vs. azurekmsWe could improve our current implementation to support multiple aliases for a single KMS type?

[maraino]

I was thinking of using aliases and, in most cases, custom parsers per kms, like the one I added in softkms and cloudkms.