Fix `MachineAuthUpdateEvent` always containing the same hash
Bump year in LICENSE.txt
Internal reorganization and update to latest SteamKit version

- This commit seperates the `internal` sub-package into multiple packages:
`protobuf` and `steamlang` which now contain the generated files and
`gamecoordinator` which is now used for all GC messages/packets.
- `protobuf` also contains a `unified` sub-package used for unified
messages
- Renamed `PacketMsg` to `Packet`.
- Added mutex for PacketHandlers in Client.
- Bonus: Improved docs
Trading: Add new fields to TradeResultEvent