Allow FCM to auto-generate typings, separate internal vs external APIs

[MathBunny]

Context

The goal is to bridge the gap between today's Admin SDK (manually curated d.ts files, nested namespaces) and a modularized SDK that has auto-generated typings.

This is the sixth milestone from: go/firebase-node-auto-typing.

Goals for this PR

1. Allow to auto-generated typings for FCM
2. Split the internal and external APIs
3. Still use the previous manually curated typings
4. Be a non-breaking change

Prerequisites

None.

Test Plan

1. Unit tests:

npm test

2. Integration tests:

export TYPE_GENERATION_MODE=nonauto && npm run integration && export TYPE_GENERATION_MODE=auto && npm run integration

3. Sanity tests:

export TYPE_GENERATION_MODE=nonauto && npm pack && ./.github/scripts/verify_package.sh firebase-* && export TYPE_GENERATION_MODE=auto && npm pack && ./.github/scripts/verify_package.sh firebase-*

[MathBunny]

Inconsistent here

[hiranya911]

Looks pretty good. Just one change suggestion. Shouldn't we also need to modify the gulp file?

[MathBunny]

Thanks for the feedback! Yes, I missed adding it to the gulpfile, that is required. Moreover, after running the sanity tests with the updated gulpfile it also uncovered a missing interface that wasn't exported 👍🏼

[hiranya911]

LGTM with a nit.