Add files support

[abraham]

• Updates uploadAttachment to api v2
• Limit publishStatus to a single mediaIds value as http doesn't support list values
• Added MultipartRequest files support to post methods

Validated with:

import 'package:mastodon_dart/mastodon_dart.dart';

const bearerToken = '';
final website = Uri.parse('https://mstdn.party');

void main() async {
  var client = Mastodon(website);
  client.token = bearerToken;

  var attachment1 = await client.uploadAttachment(
    'cat.jpg',
    description: 'cat',
  );
  print('a1 ${attachment1.id}');

  // var attachment2 = await client.uploadAttachment('cat.jpg');
  // print('a2 ${attachment2.id}');

  var response = await client.publishStatus(
    status: 'Testing with media',
    mediaIds: [attachment1.id],
  );
  print('s1 ${response.id}');

  var response2 = await client.publishStatus(status: 'Testing without media');
  print('s2 ${response2.id}');
}