secrets: support Google HSM for storage

[adamdecaf]

What were you trying to do?

Cloud HSM from Google Cloud is a hardware security module (HSM) to store encryptiuon keys and perform encryption routines with. They're common in highly secure deployments.

Usage Docs: https://cloud.google.com/kms/docs/hsm
gocloud.dev docs: https://gocloud.dev/howto/secrets/#gcp

What did you expect to see?

PayGate should support using an HSM for encryption of account numbers, and other PII data.