feat(adapter): add fetch adapter; (#6371)

lib/adapters/adapters.js

@@ -1,11 +1,13 @@
 import utils from '../utils.js';
 import httpAdapter from './http.js';
 import xhrAdapter from './xhr.js';
+import fetchAdapter from './fetch.js';
 import AxiosError from "../core/AxiosError.js";
 
 const knownAdapters = {
   http: httpAdapter,
-  xhr: xhrAdapter
+  xhr: xhrAdapter,
+  fetch: fetchAdapter
 }
 
 utils.forEach(knownAdapters, (fn, value) => {