REST handler: make API return validation opt-in

REST handler: make API return validation opt-in

It has not shown any real value in the last decade+ it was enabled,
and it can actually add quite some performance overhead. E.g., if an
API endpoint returns a few 100k of relatively simple entries we can
easily require several seconds, even tens of seconds, to run the
return validation - making it easier to run into timeouts along the
transmit path to the client.

The CLI handler has it still enabled, normally there's no timeout
there as no HTTP transmit path is involved, and d.csapak had a slight
preference for that in an off-list discussion.

The actual implementations in PMG or PVE could enable it too if
running under debug mode.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>