List all Firestore "projects" in the emulator

The RTDB UI correctly lists the multiple databases created in the emulator. But the Firestore UI only lists the "main" one, even when connecting to different project id in the emulator.

Use-case: when we run unit tests, we use the `--import` to use a common dataset for each test, but each tests starts on its own "project id" (to not contaminate other tests).
Currently it's not possible to debug those tests in the emulator UI.