[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/src/lib.rs

@@ -7,6 +7,7 @@
 mod error;
 mod local;
 mod recorder;
+mod remote;
 mod store;
 mod types;
 
@@ -14,6 +15,7 @@ pub use error::ThreadStoreError;
 pub use error::ThreadStoreResult;
 pub use local::LocalThreadStore;
 pub use recorder::ThreadRecorder;
+pub use remote::RemoteThreadStore;
 pub use store::ThreadStore;
 pub use types::AppendThreadItemsParams;
 pub use types::ArchiveThreadParams;