Feat/proxy firestore

[jozefcipa]

This PR brings a lot of changes, also some breaking ones.

Disclaimer: I suppose you won't merge this as it changes a lot of stuff but maybe some things might be useful and integrated as small PRs 🤔

• Avoid modifying firebase-admin library, now it uses Javascript Proxy instead. This should also fix the issue mentioned in the FIXME I believe
• Refactored Firebase initialization, as originally it created a new app but never really used it (as it was using new Firestore directly from @google-cloud/firestore which is initialized via an environment variable). Now it doesn't initialize anything on its own. It rather accepts a firebase app instance or uses admin.initializeApp() with reading credentials from GOOGLE_APPLICATION_CREDENTIALS (when used via CLI)
• Logging is now configurable (disabled by default) so it can be better integrated with other code
• Refactored code into several files and smaller functions to make the overall structure easier to read

[kevlened]

Thanks for opening this PR, it has some good ideas. Proxy is an interesting solution. It requires some cleverness to recursively handle the chained paths below, but still seems safer than modifying the underlying prototypes.

await firestore.doc('collection/doc').delete();
await firestore.doc('collection/doc').collection('collection').doc('doc').delete();
await firestore.collection('collection').doc('doc').collection('collection').doc('doc').delete();
// etc

I'm closing this for now, but thanks again!