Move ProtoClient to RPC crate, behind feature flag disabled in collab (#17908)

This fixes a bug where we accidentally added a `gpui` transitive
dependency in `collab`.

Release Notes:

- N/A

crates/rpc/src/rpc.rs

@@ -14,4 +14,9 @@ pub use peer::*;
 pub use proto::{error::*, Receipt, TypedEnvelope};
 mod macros;
 
+#[cfg(feature = "gpui")]
+mod proto_client;
+#[cfg(feature = "gpui")]
+pub use proto_client::*;
+
 pub const PROTOCOL_VERSION: u32 = 68;