chg: dev: Refactoring in lib/isccfg

`cfg_obj_t` objects no longer depend on the `cfg_parser_t` life-cycle; they can now persist until the last reference is detached. The `file` field, which was previously a pointer to memory allocated in the parser, is now a pointer to a subsidiary `cfg_obj_t` of type string. The API calls for creating and detaching these objects have been simplified accordingly.

Since `cfg_obj_t` is now long-lived, a zone can hold a reference to its own configuration data, making it possible to use `rndc showzone` even if `allow-new-zones` is disabled.

Several API calls related to the parser have been removed or hidden. The `cfg_parse_file()` and `cfg_parse_buffer()` functions now internally create and destroy their own parsers, eliminating the need for the caller to do so.

Most of these changes are intended to simplify dumping of running configuration data in a future commit.

Merge branch 'colin/hide-cfg-parser' into 'main'

See merge request isc-projects/bind9!11132