Expose the internal snapshot_ method for public use

### What would you like to see in the library?

Currently Firestore document triggers in EventArc are extremely clunky to use. Decoding and then parsing protobuf-encoded payloads provides a poor DX compared to the ease of accessing document data in Firebase Functions. Firebase Functions is able to provide this smooth experience by making use of an internal Firestore method, `snapshot_` found here: [snapshot_](https://github.com/googleapis/nodejs-firestore/blob/07a88420062c428b04d61638c6ba432422e01652/dev/src/index.ts#L1026)

How it's used in FF: https://github.com/firebase/firebase-functions/blob/master/src/common/providers/firestore.ts

If you expose `snapshot_` as a public method, it will make the experience of using EventArc Firestore Document triggers with something like Cloud Run way better. And since the method already exists, I can't imagine this would be such a big lift. Absolutely huge ROI on this effort for your users. Thanks!

### Describe alternatives you've considered

Painfully decoding and parsing Protobuf-encoded data

### Additional context/notes

I'm posting this here because this is where the internal snapshot_ method lives, but it would make sense to expose this through firebase-admin as well.