Recording what the Firestore emulator sends to Cloud Functions, and replaying it in Rust

hidane is a reimplementation of the Cloud Firestore emulator as a single Rust binary: no JDK, the same gRPC, REST and WebChannel wire protocols, and it runs under firebase-tools with hidane exec -- firebase emulators:start. Its rule is that every claim of compatibility has a test behind it: scripts record how the official emulator answers, and hidane's test suite replays every recording. Until now, one thing was missing under emulators:start: Firestore-triggered Cloud Functions. Since 0.2.3 hid