Consolidate logic for protobuf message handling between ssh and web socket clients (#17185)

This is a refactor to prepare for adding LSP support in SSH remote
projects.

Release Notes:

- N/A

---------

Co-authored-by: Mikayla <mikayla@zed.dev>
Co-authored-by: Conrad <conrad@zed.dev>

crates/proto/Cargo.toml

@@ -20,8 +20,10 @@ doctest = false
 anyhow.workspace = true
 collections.workspace = true
 futures.workspace = true
+parking_lot.workspace = true
 prost.workspace = true
 serde.workspace = true
+gpui.workspace = true
 
 [build-dependencies]
 prost-build.workspace = true