GCP KMS asymmetric key support

[natalysheinin]

Identical request to #684, but for GCP.

I'd like to be able to allow anyone to encrypt secrets to me; but only allow decryption through sops/KMS.
GCP support this with ASYMMETRIC_DECRYPT.

https://cloud.google.com/kms/docs/asymmetric-encryption

[brenwell]

Is this possible? I can imagine it to be quite useful.

[yz89122]

I opened a PR for this #1187