[codex] Add remote thread store implementation (#17826)

- Add a "remote" thread store implementation
- Implement the remote thread store as a thin wrapper that makes grpc
calls to a configurable service endpoint
- Implement only the thread/list method to start
- Encode the grpc method/param shape as protobufs in the remote
implementation

A wart: the proto generation script is an "example" binary target. This
is an example target only because Cargo lets examples use
dev-dependencies, which keeps tonic-prost-build out of the normal
codex-thread-store dependency surface. A regular bin would either need
to add proto generation deps as normal runtime deps, or use a
feature-gated optional dep, which this repo’s manifest checks explicitly
reject.

codex-rs/thread-store/examples/generate-proto.rs

@@ -0,0 +1,14 @@
+use std::path::PathBuf;
+
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    let proto_dir = PathBuf::from(std::env::args().nth(1).expect("proto dir"));
+    let proto_file = proto_dir.join("codex.thread_store.v1.proto");
+
+    tonic_prost_build::configure()
+        .build_client(true)
+        .build_server(true)
+        .out_dir(&proto_dir)
+        .compile_protos(&[proto_file], &[proto_dir])?;
+
+    Ok(())
+}