fix: dev: Save userconfig as text instead of a cfg_obj tree

Once the user configuration has been merged into the effective configuration, it no longer needs to be accessed as a configuration tree, but we still want to be able to show it with `rndc showconf -user`.

Because the recursive strucure of `cfg_obj` objects is fairly large, the canonical text form is a fraction of the size of the configuration tree, so we now save it in that form instead.

Merge branch 'each-userconfig-text' into 'main'

See merge request isc-projects/bind9!11185