value-pairs: Require LogTemplateOptions to be passed at run-time

Instead of storing a borrowed LogTemplateOptions copy, require passing
one in at run-time (via value_pairs_foreach() or value_pairs_walk()).
This provides more flexibility, and can guarantee thread safety too.

All drivers that use value-pairs have been updated accordingly.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
format-json: Support acting on type hints

With this patch, $(format-json) becomes able to act on type hints, and
store ints as ints and so on and so forth.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>