Add querystring support for uri mapping

[Marsup]

Hi,

I found myself in need of transmitting the query string to the proxy target, rather than redefining the mapUri, I thought it would be interesting to add it to the template variables. If it were to be accepted, I think it would need to be a breaking change, as people could have used query inside their params.

Let me know what you think.

[Marsup]

CI seems to be failing for random reasons :thinking:

[devinivy]

hapi v19 shouldn't be tested against node v16— I think that's the cause of the issues. Not sure exactly why it's like that. There is one test that's failing on hapi 19 / node 12 / ubuntu, and I expect it's just a flaky test. Will dig into a little bit. Overall I like this change and think it's worthwhile to add when we drop node v12 support on this module 👍