json: teach json builder "conditional" object fields

json: teach json builder "conditional" object fields

Quite often when we generate objects some fields should only be
generated in some conditions. Let's add high-level support for that.
Matching the existing JSON_BUILD_PAIR() this adds
JSON_BUILD_PAIR_CONDITIONAL() which is very similar, but takes an
additional parameter: a boolean condition. If "true" this acts like
JSON_BUILD_PAIR(), but if false then the whole pair is suppressed.

This sounds simply, but requires a tiny bit of complexity: when complex
sub-variants are used in fields, then we also need to suppress them.