[Net] New replication interface, spawner and synchronizer nodes.

Initial implementation of the MultiplayerReplicationInterface and its
default implementation (SceneReplicationInterface).

New MultiplayerSpawner node helps dealing with instantiation of scenes
on remote peers (e.g. clients).
It supports both custom spawns via a `_spawn_custom` virtual function,
and optional auto-spawn of known scenes via a TypedArray<PackedScenes>
property.

New MultiplayerSynchornizer helps synchronizing states between the local
and remote peers, supports both sync and spawn properties and is
configured via a `SceneReplicationConfig` resource.
It can also sync via path (i.e. without being spawned by a
MultiplayerSpawner if both peers has it in tree, but will not send the
spawn state in that case, only the sync one.

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