json: extend JsonDispatch flags with nullable and refuse-null flags

json: extend JsonDispatch flags with nullable and refuse-null flags

currently when dispatching json objects into C structs we either insist
on the field type or we don't. Let's extend this model a bit: depending
on two new fields either allow or refuse null types in addition to the
specified type.

This is useful for example when dispatch enums as this allows us
explicitly refuse null in various scenarios where we allow multiple
types.