feat: make Unity integration optional

Integration/Runtime/ShrinkEventBus.Integration.Runtime.asmdef

@@ -0,0 +1,19 @@
+{
+  "name": "ShrinkEventBus.Integration.Runtime",
+  "rootNamespace": "ShrinkEventBus.Integration",
+  "references": [
+    "ShrinkEventBus.Runtime",
+    "ShrinkRuntime.Abstractions"
+  ],
+  "autoReferenced": true,
+  "defineConstraints": [
+    "SHRINK_EVENTBUS_HAS_SHARED_RUNTIME"
+  ],
+  "versionDefines": [
+    {
+      "name": "com.cneicy.shrink-runtime-abstractions",
+      "expression": "0.1.0",
+      "define": "SHRINK_EVENTBUS_HAS_SHARED_RUNTIME"
+    }
+  ]
+}