HttpServer artifact in addition to Jetty

[ericraskin]

What the problem is:

As reported in #397, the 8.2 version of Jetty included in the google-oauth-client-jetty artifact is incompatible with newer versions of Jetty. Proposed upgrades to Jetty 9.4 require evaluation. It was suggested that an HttpServer implementation might be better than upgrading Jetty.

Describe the solution you'd like
I would like a new artifact google-oauth-client-httpserver created. I have already written the code, but it is currently under the jetty artifact, which causes confusion.

Describe alternatives you've considered
Upgrading Jetty to 9.4 is under consideration, but both options may be the best solution (upgrade Jetty AND implement an HttpServer version).