feat: make Unity integration optional

Runtime/ShrinkBusSchedulers.cs

@@ -6,7 +6,11 @@ using System.Diagnostics;
 using System.Threading;
 using System.Threading.Tasks;
 using Cysharp.Threading.Tasks;
-using ShrinkSDK.Runtime;
+#if UNITY_5_3_OR_NEWER
+using MainThreadDispatcherContract = ShrinkEventBus.IShrinkEventBusMainThreadDispatcher;
+#else
+using MainThreadDispatcherContract = ShrinkSDK.Runtime.IShrinkMainThreadDispatcher;
+#endif
 
 namespace ShrinkEventBus
 {
@@ -208,7 +212,7 @@ namespace ShrinkEventBus
     internal sealed class ShrinkMainThreadScheduler : IShrinkBusScheduler
     {
         private readonly ShrinkSchedulerQueue _queue;
-        private readonly IShrinkMainThreadDispatcher _dispatcher;
+        private readonly MainThreadDispatcherContract _dispatcher;
         private int _pumpScheduled;
         private int _disposed;