Remove auto-JSONification of query params, allowing them to be unquoted

[eddyashton]

Resolves #2153.

The json_handler adapter no longer parses the query string. Any request parameters which are included in the query string must be extracted manually by the handler, and we will not try to read/parse them beforehand. We do add some helper functions in http_query.h to remove some of the boiler plate in query parsing.

The main benefit of this is that when those query parameters are strings they no longer need to be quoted. Additionally, this gives the app more precise control of the OpenAPI description (in a form we want to extend in future). It also makes parsing of GET and POST requests more consistent and removes the requirement that we are opinionated on GETs having no body (the pre-parsed JSON will now contain the body from either method, and query strings must be manually parsed in either method).

[ghost]

non_json_query_parsing@20717 aka 20210316.28 vs main ewma over 20 builds from 20228 to 20700

[achamayou]

I think it would be good to write some unit tests for the query parsing, to cover corner cases on separators etc.

[eddyashton]

I think it would be good to write some unit tests for the query parsing, to cover corner cases on separators etc.

I've added a unit test in http_test.cpp. This confirms the behaviour of a few corner cases, let me know if you'd like anything else tested.