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

[MathBunny]

Unfortunately, I merged the previously approved #965 into the wrong branch as I forgot to change the target before merging. Otherwise, this is an identical PR.

Side-note: If you merge a PR, and then revert, you cannot re-open the PR like you can for issues (lesson learned).

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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 second milestone from: go/firebase-node-auto-typing.

Goals for this PR

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

Prerequisites

The intention for this PR is to be merged after RTDB modularization.
The gulpfile PR from hlazu-modularizing-gulpfile.

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-*

[hiranya911]

LGTM