Improve token verification logic with Auth Emulator.

[yuchenshi]

Discussion

This PR implements http://go/firebase-auth-emulator-admin-sdk (Google internal design doc). Token verification now works everywhere, not just in functions emulator.

Testing

Ran node ~/w/firebase-tools/lib/bin/firebase.js emulators:exec --project fake-project-id --only auth 'npx mocha test/integration/auth.spec.ts --slow 5000 --timeout 20000 --require ts-node/register' manually and all tests passed locally (once unrelated pending fixes in the Auth Emulator such as firebase/firebase-tools#3091 and firebase/firebase-tools#3088 are applied).

API Changes

(secret API removed; no public API changes)

[hiranya911]

Thanks @yuchenshi. The implementation looks pretty good. However, we need more work to round off testing. My suggestion is to update the CI setup to invoke the auth integ tests against the emulator as part of our regular builds.

[samtstern]

nit: took me a while staring at this diff because of the mixed use of string templates and string concatenation via +. No need to do anything though, probably not in scope.

[hiranya911]

LGTM with a suggestion. Also give @bojeil-google a chance to review before merging.