[PLAT-8049] Add bugsnag_flutter_http

[nickdowell]

Changeset

Adds the bugsnag_flutter_http package; a drop-in replacement for https://pub.dev/packages/http

Adds demonstration of usage to example app.

Usage

Import bugsnag_flutter_http instead of the http package, and use it in the same way:

import 'package: bugsnag_flutter_http/http.dart' as http;

var url = Uri.parse('https://example.com/whatsit/create');
var response = await http.post(url, body: {'name': 'doodle', 'color': 'blue'});
print('Response status: ${response.statusCode}');
print('Response body: ${response.body}');

print(await http.read(Uri.parse('https://example.com/foobar.txt')));

Testing

Includes unit tests.

Tested manually using example app.

[lemnik]

LGTM