refactor: Server pattern

[elementbound]

This is an experimental PR, that refactors much of netfox's internals, inspired by Godot's server pattern.

While Godot had its own requirements, the interpretation here is that:

• Servers are singletons that provide access to a group of related functionalities
• Servers act as an interface, similar to a facade, as an outer boundary to internal details
• Nodes act as convenient wrappers to servers, meaning they expose some config and then do some server calls based on said config

Hopefully this allows:

• Simplified history tracking
• Greatly simplified RollbackSynchronizer, StateSynchronizer, and PredictiveSynchronizer
• Easier testing
• Batching - sending state in one RPC per tick, instead of one RPC per RBS per tick
• Nested RBS's
• An easier fix for RollbackSynchronizer removes mutated states on rollback #383
• Buffering incoming data in case the target node doesn't exist yet, instead of printing errors
• Easier implementation of Rollback liveness #468

Also lays the foundations for some potentially wild features like:

• RPCs on objects
• Rollback on objects

TODO:

• Diff states
• Redundant inputs
• Binary state serialization
• Binary input serialization
• Support schemas
• Support visibility filtering
• Networked IDs
• Command bus
• StateSynchronizer
• PredictiveSynchronizer
• Update metrics
• Toggleable input broadcast
• Mutations?
• Separate into multiple PRs

Pending fixes:

• Input prediction
• Diff states

Sanity tasks:

• Implement tests
• Implement / postpone TODOs
• Test with large states, e.g. larger than the MTU of 1500 bytes
  • Large state ( 2k ) doesn't arrive consistently using noray + relay
  • Checking the same scene with 1.35.3 works well
• Test physics stepping
• Test display offset and input delay
• Test Track and expose latest confirmed tick #470
• Test all examples

Introduces #556

[Archists-dev]

Great stuff!!

[albertok]

Server always has authority to broadcast states

Suggested change

	if not subject.is_multiplayer_authority():
	if subject.multiplayer.is_server():
	return true
	if not subject.is_multiplayer_authority():

[elementbound]

Might have to investigate why is_predicting() was returning true in this case. Otherwise we risk overwriting a client's prediction with ticks that the server had no input for.

[elementbound]

Without the suggested change, RollbackSimulationServer.is_predicting() would return true for the other player's car on the server, because of this clause:

	if is_owned and not has_input:
		# We own the node, node depends on input, we don't have data for input - predict
		return true

So as long as we haven't received input from the client, we consider the recorded simulation predicted, which - in theory - makes sense. I guess next thing to find out is why we need to forego the above.

On clients, is_predicting() returns false for the player's car with this clause:

	if not is_owned and has_input:
		# We don't own the node, but we own input for it - not (input) predicting
		return false