fix: [NGOv2.x] Add OnPreShutdown to NetworkManager (#3366)

Forwardport of
#3358

This PR adds a `NetworkManager.OnPreShutdown` callback that happens
before the internal shutdown is done. This is to allow accessing any
state that is cleaned up by the `NetworkManager` during shutdown, such
as accessing dynamically spawned NetworkObjects.

## Changelog

- Added: The event NetworkManager.OnPreShutdown has been added which is
called before the NetworkManager cleans up and shuts down.

## Testing and Documentation

- A test has been added to ensure this is being called, and called
before OnServerStopped.

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### Deprecated API
- [ ] An `[Obsolete]` attribute was added along with a `(RemovedAfter
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- [ ] Deprecation of the API is explained in the CHANGELOG.
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---------

Co-authored-by: Noel Stephens <[email protected]>

Author: jonathanhertz

com.unity.netcode.gameobjects/CHANGELOG.md

@@ -10,6 +10,7 @@ Additional documentation and release notes are available at [Multiplayer Documen
 
 ### Added
 
+- Added `NetworkManager.OnPreShutdown` which is called before the NetworkManager cleans up and shuts down. (#3366)
 - Added interpolator types as an inspector view selection for position, rotation, and scale. (#3337)
 - Added a new smooth dampening interpolator type that provides a nice balance between precision and smoothing results. (#3337)
 - Added `NetworkTimeSystem.TickLatency` property that provides the average latency of a client. (#3337)

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

@@ -807,6 +807,12 @@ public struct ConnectionApprovalRequest
         /// </summary>
         public event Action OnClientStarted = null;
 
+        /// <summary>
+        /// Subscribe to this event to get notifications before a <see cref="NetworkManager"/> instance is being destroyed.
+        /// This is useful if you want to use the state of anything the NetworkManager cleans up during its shutdown.
+        /// </summary>
+        public event Action OnPreShutdown = null;
+
         /// <summary>
         /// This callback is invoked once the local server is stopped.
         /// </summary>
@@ -1481,6 +1487,8 @@ internal void ShutdownInternal()
                 NetworkLog.LogInfo(nameof(ShutdownInternal));
             }
 
+            OnPreShutdown?.Invoke();
+
             this.UnregisterAllNetworkUpdates();
 
             // Everything is shutdown in the order of their dependencies

com.unity.netcode.gameobjects/Tests/Runtime/NetworkManagerEventsTests.cs

@@ -236,6 +236,46 @@ public IEnumerator OnClientAndServerStartedCalledWhenHostStarts()
             Assert.AreEqual(2, callbacksInvoked, "either OnServerStarted or OnClientStarted wasn't invoked");
         }
 
+        [UnityTest]
+        public IEnumerator OnPreShutdownCalledWhenShuttingDown()
+        {
+            bool preShutdownInvoked = false;
+            bool shutdownInvoked = false;
+            var gameObject = new GameObject(nameof(OnPreShutdownCalledWhenShuttingDown));
+            m_ServerManager = gameObject.AddComponent<NetworkManager>();
+
+            // Set dummy transport that does nothing
+            var transport = gameObject.AddComponent<DummyTransport>();
+            m_ServerManager.NetworkConfig = new NetworkConfig() { NetworkTransport = transport };
+
+            Action onPreShutdown = () =>
+            {
+                preShutdownInvoked = true;
+                Assert.IsFalse(shutdownInvoked, "OnPreShutdown was invoked after OnServerStopped");
+            };
+
+            Action<bool> onServerStopped = (bool wasAlsoClient) =>
+            {
+                shutdownInvoked = true;
+                Assert.IsTrue(preShutdownInvoked, "OnPreShutdown wasn't invoked before OnServerStopped");
+            };
+
+            // Start server to cause initialization process
+            Assert.True(m_ServerManager.StartServer());
+            Assert.True(m_ServerManager.IsListening);
+
+            m_ServerManager.OnPreShutdown += onPreShutdown;
+            m_ServerManager.OnServerStopped += onServerStopped;
+            m_ServerManager.Shutdown();
+            Object.DestroyImmediate(gameObject);
+
+            yield return WaitUntilManagerShutsdown();
+
+            Assert.False(m_ServerManager.IsListening);
+            Assert.True(preShutdownInvoked, "OnPreShutdown wasn't invoked");
+            Assert.True(shutdownInvoked, "OnServerStopped wasn't invoked");
+        }
+
         private IEnumerator WaitUntilManagerShutsdown()
         {
             /* Need two updates to actually shut down. First one to see the transport failing, which