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/fetchProxy/FetchInput.ts b/src/xhrInterceptor/fetchProxy/FetchInput.ts
new file mode 100644
index 0000000..75be8b8
--- /dev/null
+++ b/src/xhrInterceptor/fetchProxy/FetchInput.ts
@@ -0,0 +1,16 @@
+export default class FetchInput {
+  constructor(private input: RequestInfo | URL) {}
+
+  /**
+   * Returns the fetch input URL as a string.
+   */
+  getUrl() {
+    if (typeof this.input === 'string') {
+      return this.input;
+    } else if (this.input instanceof URL) {
+      return this.input.toString();
+    } else {
+      return this.input.url;
+    }
+  }
+}