[Firestore] Add ability to quickly switch to a map

[cedvdb]

When decoding a document to a map, the map coming from the data() method is not agnostic of firestore. At the moment I'm using this to decode data, so firestore doesn't leak to the rest of my program:

  /// helper to convert a document snapshot to a map like a map we'd receive
  /// after json decoding a normal API
  Map<String, dynamic> docToMap(DocumentSnapshot doc) {
    Map<String, dynamic> map = Map();
    final data = doc.data();

    if (data == null) return null;

    map['id'] = doc.id;
    data.entries.forEach((entry) {
      var value = entry.value;
      // if the value is a google timestamp we convert it to a datetime
      if (value is Timestamp) {
        value = value.toDate();
      }
      map[entry.key] = value;
    });
    return map;
  }

I just think it would be nice if that was built in, this fits my usecase, but I'm sure there are more types to check than just timestamp. I just don't think back end types should leak into the frontend.