Merged requested to merge manu/olmkit_pickling_v2 into master
Improve ObjC wrappers so that they can use a pickle key provided by the olm lib user.
This new behavior is optional to not break existing usage.
It is retro compatible and use pickle versioning already in place. Existing key will be unpickled with pickle v1 and pickled with pickle v2 if an external pickle key is provided.