✨ add per-request timeout

[techouse]

This PR adds a new timeout annotation

/// Set a timeout for the request
final Duration? timeout;

that can be used like

@Get(path: 'get_timeout', timeout: Duration(seconds: 42))
Future<Response<String>> getTimeoutTest();

and generates the following code

@override
Future<Response<String>> getTimeoutTest() {
  final Uri $url = Uri.parse('/test/get_timeout');
  final Request $request = Request(
    'GET',
    $url,
    client.baseUrl,
  );
  return client
      .send<String, String>($request)
      .timeout(const Duration(microseconds: 42000000));
}

---

Addresses #603
Addresses dart-lang/http#1186

[techouse]

Codecov complaining .... as always.

@JEuler @Guldem @vladRudenua23 would you mind checking this PR 😇

[Guldem]

LGTM! Nice addition