Support for RSA4096 / SHA512

[SgtCoDFish]

The cert-manager project would like to use GCP KMS keys when signing helm charts. Helm currently hardcodes a requirement to use SHA512 digests, though, so this library can't be used directly as-is.

I've got a local development branch with changes to google-kms-pgp which enables the use of the RSA_SIGN_PKCS1_4096_SHA512 KMS key type. This involves bumping the version of cloud.google.com/go and several smallish code changes to support the different digest type.

I'd be happy to clean up the local code and raise a PR which would add this support upstream, so everyone can benefit. I'm raising this issue first to check if such a PR would be appreciated before I do the extra cleanup work, and because there hasn't been much activity on this repo recently I wanted to check if anyone was maintaining it 😁