Removed the `Firestore.Encoder` and `Firestore.Decoder` encoding/decoding strategy type aliases and directly used the types from `FirebaseDataEncoder` and `FirebaseDataDecoder`.
Note: This requires the `TimestampEncoding/DecodingStrategy` extension to be added to `FirebaseDataEncoder/Decoder` instead of `Firestore.Encoder` and `Firestore.Decoder`. This will make the `timestamp` strategy visible to all users of the `FirebaseDataEncoder/Decoder` (that have FirestoreSwift installed), even though `Timestamp` is a Firestore-specific type.