Expose generic types of @google-cloud/firestore

[mamichels]

This exposes the already existing generic types of @google-cloud/firestore for DocumentSnapshot and QueryDocumentSnapshot.
I've also added the default to not introduce any breaking change and keep the typing optional. In regard to less readable documentation and a cleaner implementation I've also added the default type DocumentData as an exposed type.

Currently you cannot even overwrite QueryDocumentSnapshot with the generic type of @google-cloud/firestore since the handler will not accept the same as it uses an internal reflected type of QueryDocumentSnapshot. Hence an explicit type cast needs to be placed during unwrapping. E.g.

import { firestore } from './src';
import { EventContext } from './src/cloud-functions';
import { QueryDocumentSnapshot } from './src/providers/firestore';

interface MyDocument {
    foo: string;
}

export const firebaseFunction = firestore
    .document('{collection}/{id}')
    .onCreate(async (data: QueryDocumentSnapshot, context: EventContext) => {
        // I don't want that 😥
        console.log('data: ' + (data.data() as MyDocument).foo);
    });

This change will enable the following implementation:

import { firestore } from './src';
import { EventContext } from './src/cloud-functions';
import { QueryDocumentSnapshot } from './src/providers/firestore';

interface MyDocument {
    foo: string;
}

export const firebaseFunction = firestore
    .document('{collection}/{id}')
    .onCreate(async (data: QueryDocumentSnapshot<MyDocument>, context: EventContext) => {
        // It works! 🥰
        console.log('data: ' + data.data().foo);
    });

If you need me to make any changes please let me know.