fix(admin-firestore): add types from @google-cloud/firestore

[mkorenko]

When initializing an app using firebase-admin with admin.firestore() instead of firebase.firestore():

import * as functions from 'firebase-functions';
admin.initializeApp(functions.config().firebase);
const firestore = admin.firestore();
firestorm.initialize(firestore);

getting:

1. ts error error TS2345: Argument of type 'FirebaseFirestore.Firestore' is not assignable to parameter of type 'firebase.firestore.Firestore'.
2. after fixing it, getting runtime error for @timestamp() and @geopoint() fields : TypeError: Cannot read property 'Timestamp' of undefined

I guess that happens because different types are imported by non-admin and admin SDKs:
from firebase/index.d.ts and @google-cloud/firestore/types/firestore.d.ts respectively.

Fixed that for TimeStamps and GeoPoints using combined types/fallback refs, and also types in firestorm.initialize()

[Legogris]

Fails in CI:

src/firestore.ts
ES2015 module syntax is preferred over custom TypeScript modules and namespaces. (@typescript-eslint/no-namespace)

export namespace firestore {

[Mistic92]

Any update?:)