GCP KMS for Key Provider for Encryption

[cam72cam]

Initial manual testing performed, unit tests required, potential storage of AAD in metadata.

Resolves #1173

Example:

terraform {
        encryption {
                key_provider "gcp_kms" "basic" {
                        kms_encryption_key = "projects/local-vehicle-redacted/locations/global/keyRings/opentofu-test/cryptoKeys/opentofu-keytest"
                        credentials = "/path/to/creds.json"
                        key_size = 32
                }
                method "aes_gcm" "example" {
                        keys = key_provider.gcp_kms.basic
                }
                state {
                        method = method.aes_gcm.example
                }
        }
}

resource "tfcoremock_simple_resource" "simple" {
        string = "helloworld changes! 5"
}

Target Release

1.7.0

[github-actions]

Reminder for the PR assignee: If this is a user-visible change, please update the changelog as part of the PR.

[cube2222]

Generally, lgtm!

[janosdebugs]

LGTM, one minor nitpick