fix: add FetchProxy

This will let us intercept fetch requests (until now we're only proxying
XMLHttpRequest), in order to fix the issues we're experiencing with some
features.

Bug: twpowertools:229
Change-Id: I277473c05479ca39bb6183a51855382124890bde
diff --git a/src/xhrInterceptor/MessageIdTracker.ts b/src/xhrInterceptor/MessageIdTracker.ts
new file mode 100644
index 0000000..4540b23
--- /dev/null
+++ b/src/xhrInterceptor/MessageIdTracker.ts
@@ -0,0 +1,7 @@
+export default class MessageIdTracker {
+  private messageId = 0;
+
+  getNewId() {
+    return this.messageId++;
+  }
+}