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/remote_server/src/headless_project.rs

@@ -8,8 +8,8 @@ use project::{
 };
 use remote::SshSession;
 use rpc::{
-    proto::{self, AnyProtoClient, SSH_PEER_ID, SSH_PROJECT_ID},
-    TypedEnvelope,
+    proto::{self, SSH_PEER_ID, SSH_PROJECT_ID},
+    AnyProtoClient, TypedEnvelope,
 };
 use smol::stream::StreamExt;
 use std::{