Allow to duplicate GrpcMethod extensions (#1425)

This is required for (manually) cloning requests, which in turn is
required for middlewares that retries requests on demand.

tonic/src/extensions.rs

@@ -73,7 +73,7 @@ impl fmt::Debug for Extensions {
 }
 
 /// A gRPC Method info extension.
-#[derive(Debug)]
+#[derive(Debug, Clone)]
 pub struct GrpcMethod {
  service: &'static str,
  method: &'static str,