1. Rust:
Run `cargo build` from `services/tunnelbroker/rust-notifications` directory.
Rust library will be successfully built.
2. C++ (Docker):
Patch to D4807 (top of the stack) using `arc patch D4807`.
Running `run run-tunnelbroker-service` successfully built the Rust library and link it.
3. C++ (Nix):
Patch to D4807 (top of the stack) using `arc patch D4807`.
Call `cd services/tunnelbroker`.
Running `rm -dfr build && cmake -B build . && make -C build -j4` successfully built the Rust library and link it.
Library's FCM notifications push method can be called from the C++ as:
```
#include "RustNotifications.h"
...
const bool result = sendNotifToFCM(
char const *fcmAPIKey,
char const *deviceRegistrationID,
char const *messageTitle,
char const *messageBody
);
```