CP-2964 Add ability to "record" HTTP requests and WebSocket connections

[evanweible-wf]

Example usage:

final recording = MockTransports.http.record();

final uri = Uri.parse('http://httpstat.us/200');
await Http.get(uri);
await Http.post(uri);

recording.stop();

expect(recording.requests.first.method, equals('GET'));
expect(recording.requests.first.uri, equals(uri));

expect(recording.requests.last.method, equals('POST'));
expect(recording.requests.last.uri, equals(uri));