chg: dev: Make dns_glue_t private to qpzone

The dns_glue struct currently contains four dns_rdataset structs to hold
the glue. These structs are over 100 bytes each because they need to be
able to hold data for multiple types of databases.

Since the dns_glue_t type is only used by qpzone, we can instead hold pointers
to the vecheaders directly, and only bind the vecheaders to the
rdatasets when adding the glue to the message.

This leads to a 33% memory reduction in some authoritative benchmarks.

Merge branch 'alessio/glue-ozempic' into 'main'

See merge request isc-projects/bind9!11801