fix: dev: Reduce the size of cfg_obj_t

Instead of having `isc_sockaddr`, `isc_netaddr`, and `isccfg_duration` members in the `cfg_obj->value`
union, we now just keep pointers to them, and allocate memory when parsing these types. This reduces the
size of `cfg_obj_t` from 112 bytes to 72.

Merge branch 'each-refactor-cfg-obj' into 'main'

See merge request isc-projects/bind9!11186