Functions emulator matches too many URLs

[samtstern]

[REQUIRED] Environment info

firebase-tools: 7.6.1

Platform: all

[REQUIRED] Test case

See #1771 for original report

[REQUIRED] Steps to reproduce

Ping localhost:5001/foo/bar/baz

[REQUIRED] Expected behavior

Ignored because that's not a Firebase-relevant URL.

[REQUIRED] Actual behavior

This line in functionsEmulator.ts:

const httpsFunctionRoute = `/:project_id/:region/:trigger_name`;

Matches basically any three-segment URL. So the above would try to run function baz in region bar in project foo.

We should do a few things:

1. When initializing the functions emulator, hard-code the projectId into this path.
2. When running the HTTPs handler, make sure that region is one of a valid set before trying to execute a function, otherwise send a 4xx error.