[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/Cargo.toml

@@ -347,6 +347,8 @@ tracing-appender = "0.2.3"
 tracing-opentelemetry = "0.32.0"
 tracing-subscriber = "0.3.22"
 tracing-test = "0.2.5"
+tonic = { version = "0.14.3", default-features = false, features = ["channel", "codegen"] }
+tonic-prost = "0.14.3"
 tree-sitter = "0.25.10"
 tree-sitter-bash = "0.25"
 ts-rs = "11"