Investigate implementation of a catch-all json member

[beached]

Name of mapping : TBD
Purpose: To provide a json_value like structure over the unmapped members in a class. Because we know the mapped members and their locations, that location_info structure can be used as a filter list and is of fixed size.

example usage:

{
  "a": 1,
  "b": 2,
  "c": 3 
}

With a mapping like

using type = json_member_list<json_link<"b", int>, json_catch_all>;

Would require a class that is constructible from an int, json_catch_all_value<1>

The 1 is because the size of the filter list is 1 element.