[chopper_generator] Add types to the generated variables

[techouse]

While I was migrating expression.assignFinal(name) to declareFinal(name).assign(expression) I noticed that I could also add types in there, however, it did not feel appropriate for the PR #358.

So, I'm adding them here, which in essence makes code like this

@override
Future<Response<String>> getTest(
  String id, {
  required String dynamicHeader,
}) {
  final $url = '/test/get/${id}';
  final $headers = {
    'test': dynamicHeader,
  };

  final $request = Request(
    'GET',
    $url,
    client.baseUrl,
    headers: $headers,
  );
  return client.send<String, String>($request);
}

look like

@override
Future<Response<String>> getTest(
  String id, {
  required String dynamicHeader,
}) {
  final String $url = '/test/get/${id}';
  final Map<String, String> $headers = {
    'test': dynamicHeader,
  };
  final Request $request = Request(
    'GET',
    $url,
    client.baseUrl,
    headers: $headers,
  );
  return client.send<String, String>($request);
}

Compare the previous chopper/test/test_service.chopper.dart to the one in this PR to see more changes.