Add support for DocumentReference

[Barbacamanitu]

I just tried firedantic out today and did not see any way to include a DocumentReference to a model, but Firestore documents can contain references to other documents. Sure, you could just use an id, but I'm working with a Firestore database set up by someone else and they use references everywhere so I have to support them if I'm going to model their data.

Before I knew about firedantic, I set up support for these myself using a wrapper type containing simply "path" and "id", which serializes and deserializes to/from firestore. I'll submit a PR when I get a chance to add this in.

Again, if there is already support for this somewhere, where is it? The Readme doesn't mention references at all.

[joakimnordling]

You are right, there is no support for this in firedantic yet.

[Barbacamanitu]

I have a working implementation of DocumentReferences and even TypedDocumentReferences that correctly grab objects from firestore based on the reference type. I'm going to clean the code up a bit and create a PR soon.