Adding support for <product>_EMULATOR_BINARY_PATH for local emualtor dev

[joehan]

Description

Added a new environment variable <product>_EMULATOR_BINARY_PATH that tells the emulator suite to skip downloading/checking a downloadable emulator and instead use the binary at the provided path.

Sample Commands

// Replace FIRESTORE with any other product, or UI
export FIRESTORE_EMULATOR_BINARY_PATH=path/to/firestore/emulator

// And the emulator suite will use your local version at that path
firebase emulators:start --only firestore