[WIP] Refactor and support encoding to timestamp and decoding to date

[caoer]

I'd like to keep the model clean, without any dependency from firestore, which limits us to use Date, and not Timestamp. This PR adds an ability of it.

I also refactor and clean the code a bit.

I haven't use and test other types. gonna work on them later...

[caoer]

FirestoreEncoder(userInfo: [
            CodingUserInfoKey.dateEncodingStrategy:
                DateEncodingStrategy.deferredToTimestamp({ Timestamp(date: $0) })
            ])

FirestoreDecoder(userInfo: [
            CodingUserInfoKey.dateDecodingStrategy:
                DateDecodingStrategy.deferredToTimestamp
            ])

[alickbass]

Hi! Thanks a lot for working on this! TBH, this Pull request is massive, it is really hard to review it. Could you please split it to smaller pull requests with more concrete goals? And about cleaning up, the way the code is organized now is intentional, to reflect the structure of the JSONEncoder and JSONDecoder in apple/swift, so I would prefer to keep it as close to that as possible

[agordeev]

It'd be great to see this merged so we could map Date to Timestamp

[jimijon]

CAN WE GET THIS INTO MASTER?

[inquisitiveSoft]

This looks awesome. Any chance we could get this merged?
It's not that long, considering most of it is pretty boilerplate-y.

For now I think I'll make a fork, because this is such a big quality of life improvement for me

[bradleymackey]

Bump