Allow RTDB 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 first milestone from: go/firebase-node-auto-typing.

Goals for this PR

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

To verify that the typings work as expected:

export TYPE_GENERATION_MODE=auto && npm test && npm run integration && npm pack && ./.github/scripts/verify_package.sh firebase-admin-*

I also manually ran npm pack and verify_script.sh, however for npm pack it requires manual modification of the package.json so that it invokes the proper gulp build script.

Several concerns:

1. Should I make the FirebaseApp split into an interface and class (similar to App today)?
2. I added an extra admin in the index.ts's declared namespace for database (as compared to the Auth POC) because otherwise when installing firebase-admin the current index.d.ts wouldn't work. For example:

declare namespace admin.database {
  export import Database = _database.admin.database.Database;
  // this would have to be _database.database.Database; but right now it's _database.admin.database.Database
  // ...
}

3. It shouldn't be necessary to change the integration tests at all; this is because RTDB typings are using 'unknown' for val()'s return type right now. I asked Sebastian about his input on this.

I imported the database types for the DataSnapshot, is that ok?

[hiranya911]

Overall shape looks good. But lets clean up the gulp script a bit and also figure out the discrepancy between database and database-types packages. And lets also try to narrow down the scope of the changes by keeping the FirebaseApp module unchanged in this PR.

[hiranya911]

This should come from the database-types package as well I think. In fact all RTDB types should come from the database-types package.

[hiranya911]

Looks pretty good. I think there are few places where we can import our augmented Database type instead of the FirebaseDatabase type. Can you also comment on why the interop-types package was needed?

[MathBunny]

Thanks for the feedback. Regarding the interop-types, turns out I don't actually need it, thanks for pointing that out.

[hiranya911]

Thanks. LGTM 👍