fix(signer-gcp): use default public key format (#2217)

Author: shekhirin

crates/signer-gcp/src/signer.rs

@@ -231,7 +231,11 @@ async fn request_get_pubkey(
     client: &Client,
     kms_key_name: &str,
 ) -> Result<PublicKey, GcpSignerError> {
-    let mut request = tonic::Request::new(GetPublicKeyRequest { name: kms_key_name.to_string() });
+    let mut request = tonic::Request::new(GetPublicKeyRequest {
+        name: kms_key_name.to_string(),
+        // When not specified, the default will be used.
+        public_key_format: Default::default(),
+    });
     request
         .metadata_mut()
         .insert("x-goog-request-params", format!("name={}", &kms_key_name).parse().unwrap());