Implement @Codable integrations for encoding and decoding to and from Firestore

[bburgess19]

Overview

Encoding and decoding data to and from Firestore is one of the most common operations in the Firebase SDK. The IOS SDK provides lovely integrations to that end via @Codable, but the implementation current implementation in SkipFirestore doesn't have the same.

It would be a great convenience to implement these integrations.

Current Workaround

It is possible to work around these limitations by manually decoding the data dictionaries returned, but this is more tedious than the native implementation.