feat(helpers) new busted assertions and modifiers (Kong#1322)

* add Busted/Luassert assertions and modifiers for request handling
* rework comments tbo, and updated some documentation comments
* remove custom properties, rearranged admin and proxy helpers.
* insert place holder for `res_status` assertion, will break too many open pr's if changed now.
* fix the `fail` assertion to properly display all input (instead of only string values)