before changing the JSON backend, add a test to confirm how unicode is being serialized
switch JSON backend from JSON.pm to JSON::MaybeXS, and allow additional options to be passed to the JSON backend

JSON.pm only supports JSON::PP and JSON::XS, whereas there is another very
popular backend now, Cpanel::JSON::XS.

The new "serializer_options" config allow customizing the behaviour of the JSON
serializer - e.g. changing the options for utf8, ascii, pretty, allow_nonref,
etc.

*There is no change* in how unicode is treated by default.