refactor: snapshot prep for RPCs and value update (#1520)

* experimental: Snapshot revisiting basis

* Adding back minimal ticking and update code

* Adding back minimal ticking and update code

* random experimentation

* progressing on new structure

* Debugging information

* test: add sanity test for ticksystem

* fix time system init

* adding code to read/write the spawns

* spawn snapshot, redone, still in-progress

* despawns

* fixing despawn lookup of tick applied

* tests fixed. Now need to integrate with transport correctly

* adding the SnapshotSystem's update of client/server data

* Going back to passing SnapshotDataMessage around, but using lightweight pre-serialized messages

* working for test and some initial SampleScene

* growing buffers on the send side, as needed

* stable, up-to buffer overflow

* using header, starting to put back the ClientData

* snapshot spawn/despawn refactored

* reverting unneeded change to NetworkManager

* style: code review comments

* style: minor review comments

Co-authored-by: Luke Stampfli <luke@stampfli.one>

Author: jeffreyrainy

com.unity.netcode.gameobjects/Runtime/Core/NetworkObject.cs

@@ -404,8 +404,6 @@ private SnapshotDespawnCommand GetDespawnCommand()
             var command = new SnapshotDespawnCommand();
             command.NetworkObjectId = NetworkObjectId;
 
-            command.NetworkObject = this;
-
             return command;
         }
 
@@ -434,37 +432,36 @@ private SnapshotSpawnCommand GetSpawnCommand()
             command.ObjectRotation = transform.rotation;
             command.ObjectScale = transform.localScale;
 
-            command.NetworkObject = this;
-
             return command;
         }
 
         private void SnapshotSpawn()
         {
             var command = GetSpawnCommand();
-            NetworkManager.SnapshotSystem.Spawn(command);
+            NetworkManager.SnapshotSystem.Spawn(command, this, null);
         }
 
         private void SnapshotSpawn(ulong clientId)
         {
             var command = GetSpawnCommand();
-            command.TargetClientIds = new List<ulong>();
-            command.TargetClientIds.Add(clientId);
-            NetworkManager.SnapshotSystem.Spawn(command);
+            var targetClientIds = new List<ulong>();
+            targetClientIds.Add(clientId);
+
+            NetworkManager.SnapshotSystem.Spawn(command, this, targetClientIds);
         }
 
         internal void SnapshotDespawn()
         {
             var command = GetDespawnCommand();
-            NetworkManager.SnapshotSystem.Despawn(command);
+            NetworkManager.SnapshotSystem.Despawn(command, this, null);
         }
 
         internal void SnapshotDespawn(ulong clientId)
         {
             var command = GetDespawnCommand();
-            command.TargetClientIds = new List<ulong>();
-            command.TargetClientIds.Add(clientId);
-            NetworkManager.SnapshotSystem.Despawn(command);
+            var targetClientIds = new List<ulong>();
+            targetClientIds.Add(clientId);
+            NetworkManager.SnapshotSystem.Despawn(command, this, targetClientIds);
         }
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]