Allow rpc_url to be assigned on Client with test-support feature (#13430)

Also, allow proto messages to be deserialized. This is to support
translating these messages JS types in a new server implementation based
on CloudFlare durable objects.

Release Notes:

- N/A

crates/proto/src/proto.rs

@@ -8,7 +8,7 @@ pub use error::*;
 pub use typed_envelope::*;
 
 use collections::HashMap;
-pub use prost::Message;
+pub use prost::{DecodeError, Message};
 use serde::Serialize;
 use std::any::{Any, TypeId};
 use std::time::Instant;