[1.21.4] Add optional registries & refactor registry sync

[modmuss50]

• General code cleanup, a lot of stuff can now be made client only as we no longer persist ids.
  • There is scope to move even more (such as the registry remapping code) to the client, but ill leave this for later.
• Add some more unit tests :)
• Add optional registries

Optional registries / stricter defaults

The goal of these changes is to document the expected behaviour of missing registry's when a client connects to a modded server. This behaviour is now consistent between a vanilla clients and modded clients. This has nothing to do with individual registry entries (yet).

Some mods have their own registries that only need to be synced when the connecting client has their mod, I have added a new OPTIONAL registry attribute that allows this. This attribute is sent in the registry sync packet.

The behaviour of a connecting client is as follows:

These changes will only apply to 1.21.4 as they are breaking.

TODO

• Test with real clients and servers
• Better error message when a registry is missing, I just reused the entries one for now?