Enable http logging for flutter

[yongjhih]

• [x] I have added at least 1 unit test in Dart for the new API and made sure the tests pass locally.

• Added InstabugHttpOverrides to enable http logging easily.

• Simplized InstabugCustomClient

  • replaced Request.close() with Request.done(), it avoids to get an exception of request has been closed if somewhere is still using.
  • Avoid memory leaking risks for the logger, the HttpClientLogger doesn't need to record requests for logging.