feat(adapter): add fetch adapter;

[DigitalBrainJS]

The goal of this PR is to add a minimal fetch implementation that provides the same capabilities as the xhr adapter without major changes, allowing it to be released in Axios 1.x.
To avoid breaking changes, by default fetch adapter will be used if the xhr and http adapters are not available. The user can explicitly select it in the config if needed.

const url = 'https://httpbin.org/post';

const form = new FormData();
form.append('foo', '123');

const {data} = await axios.post(url, form, {
   adapter: 'fetch',
   params: {
      foo: 1,
      bar: 2
   },
   onUploadProgress(e) {
      console.log('Upload:', e);
   },
   onDownloadProgress(e) {
      console.log('Download:', e);
   }
});