message: Make `withCredentials` public in GCPKmsClient
  This allows to instantiate a new KMSClient using GoogleCredential used elsewhere in the app.
diff: @@ -108,7 +108,7 @@ public final class GcpKmsClient implements KmsClient {
  }
  /** Loads the provided credential. */
  - private KmsClient withCredentials(GoogleCredential credential) {
  + public KmsClient withCredentials(GoogleCredential credential) {
  if (credential.createScopedRequired()) {
  credential = credential.createScoped(CloudKMSScopes.all());
  }
  