DoH: Add isc_nm_set_min_answer_ttl()

DoH: Add isc_nm_set_min_answer_ttl()

This commit adds an isc_nm_set_min_answer_ttl() function which is
intended to to be used to give a hint to the underlying transport
regarding the answer TTL.

The interface is intentionally kept generic because over time more
transports might benefit from this functionality, but currently it is
intended for DoH to set "max-age" value within "Cache-Control" HTTP
header (as recommended in the RFC8484, section 5.1 "Cache
Interaction").

It is no-op for other DNS transports for the time being.