feat: [Backport] Expose methods for ClientId to TransportId mappings (#3515)

[MTTB-950](https://jira.unity3d.com/browse/MTTB-950)

implements: #2668 
fixes: #2359 

## Changelog

- Added: Mappings for ClientId to TransportId

## Testing and Documentation

- Includes unit tests.

## Backport

Backport of #3516

---------

Co-authored-by: Patrick Coglan <coglanpatrick@icloud.com>

Author: EmandM

com.unity.netcode.gameobjects/CHANGELOG.md

@@ -10,6 +10,7 @@ Additional documentation and release notes are available at [Multiplayer Documen
 
 ### Added
 
+- Added mappings between `ClientId` and `TransportId`. (#3515)
 - Added `SinglePlayerTransport` that provides the ability to start as a host for a single player network session. (#3475)
 - When using UnityTransport >=2.4 and Unity >= 6000.1.0a1, SetConnectionData will accept a fully qualified hostname instead of an IP as a connect address on the client side. (#3440)
 

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

@@ -1150,6 +1150,20 @@ private void HostServerInitialize()
             ConnectionManager.InvokeOnClientConnectedCallback(LocalClientId);
         }
 
+        /// <summary>
+        /// Get the TransportId from the associated ClientId.
+        /// </summary>
+        /// <param name="clientId">The ClientId to get the TransportId from</param>
+        /// <returns>The TransportId associated with the given ClientId</returns>
+        public ulong GetTransportIdFromClientId(ulong clientId) => ConnectionManager.ClientIdToTransportId(clientId);
+
+        /// <summary>
+        /// Get the ClientId from the associated TransportId.
+        /// </summary>
+        /// <param name="transportId">The TransportId to get the ClientId from</param>
+        /// <returns>The ClientId from the associated TransportId</returns>
+        public ulong GetClientIdFromTransportId(ulong transportId) => ConnectionManager.TransportIdToClientId(transportId);
+
         /// <summary>
         /// Disconnects the remote client.
         /// </summary>
@@ -1310,7 +1324,7 @@ private void OnDestroy()
             }
 #if UNITY_EDITOR
             EditorApplication.playModeStateChanged -= ModeChanged;
-#endif            
+#endif
         }
 
         // Command line options

testproject/Assets/Tests/Runtime/NetworkManagerTests.cs

@@ -9,9 +9,28 @@
 
 namespace TestProject.RuntimeTests
 {
+    public class NetworkManagerTests : NetcodeIntegrationTest
+    {
+        protected override int NumberOfClients => 2;
+
+        [Test]
+        public void ValidateTransportAndClientIds()
+        {
+            var transportId = m_ServerNetworkManager.GetTransportIdFromClientId(m_ServerNetworkManager.LocalClientId);
+            Assert.IsTrue(m_ServerNetworkManager.GetTransportIdFromClientId(m_ServerNetworkManager.LocalClientId) == m_ServerNetworkManager.ConnectionManager.ServerTransportId);
+            Assert.IsTrue(m_ServerNetworkManager.GetClientIdFromTransportId(transportId) == m_ServerNetworkManager.LocalClientId);
+
+            foreach (var client in m_ClientNetworkManagers)
+            {
+                transportId = m_ServerNetworkManager.GetTransportIdFromClientId(client.LocalClientId);
+                Assert.AreEqual(client.LocalClientId, m_ServerNetworkManager.GetClientIdFromTransportId(transportId), "Server and client transport IDs don't match.");
+            }
+        }
+    }
+
     [TestFixture(UseSceneManagement.SceneManagementDisabled)]
     [TestFixture(UseSceneManagement.SceneManagementEnabled)]
-    public class NetworkManagerTests : NetcodeIntegrationTest
+    public class NetworkManagerSceneTests : NetcodeIntegrationTest
     {
         private const string k_SceneToLoad = "InSceneNetworkObject";
         protected override int NumberOfClients => 0;
@@ -33,7 +52,7 @@ public enum UseSceneManagement
 
         private bool m_UseSceneManagement;
 
-        public NetworkManagerTests(UseSceneManagement useSceneManagement)
+        public NetworkManagerSceneTests(UseSceneManagement useSceneManagement)
         {
             m_UseSceneManagement = useSceneManagement == UseSceneManagement.SceneManagementEnabled;
         }