Have one mechanism to produce json with sorted keys instead of N different ways

[mkustermann]

Currently there's many different places in the code that look like this:

  Map<String, Object?> toJson() => {
        for (final key in encoding.keys) key: encoding[key],
        _typeKey: type,
      }..sortOnKey();

Often this property isn't even used. But when it's used it's problematic because:

• it's error prone: As every single toJson() implementation would need to remember this and it's easy to forget to add this
• it's redundant code: We have this in many different places.

I suggest we centralize this into one mechanism that can be generally used. A user that requires keys to be sorted can then call it and is guaranteed to get recursively sorted maps in the entire json, e.g.

Map<String, Object?> json;

final jsonSorted = ensureSortedJsonMaps(json); // <-- recursively walks the tree, sorting any maps when needed