new: dev: introduce cfg_obj_clone to clone a config tree

Introduce `cfg_obj_clone` which takes a `cfg_obj_t` node and clones it.
it allocates a new node, copies its scalar values and recursively
allocates child nodes, copying their scalar values as well and so on.

Internally, a new method `cfg_copyfunc_t` copy is added in `cfg_rep_t`,
which enables implementing a copy function specific for each
representation type a node can hold.

This is pre-require work for MR !11121 !11122 !11123

Merge branch 'colin/effective-config-clone' into 'main'

See merge request isc-projects/bind9!11124