fix: nil: Add new convenience functions to classify rdata types

- `dns_rdatatype_ismulti()` returns true if a given type can have
  multiple answers: ANY, RRSIG, or SIG.
- `dns_rdatatype_issig()` returns true for a signature: RRSIG or SIG.
- `dns_rdatatype_isaddr()` returns true for an address: A or AAAA.
- `dns_rdatatype_isalias()` returns true for an alias: CNAME or DNAME.

Code has been modified to use these functions where applicable.

These and all similar functions (e.g., `dns_rdatatype_ismeta()`, `dns_rdatatype_issingleton()`, etc) are now `static inline` functions defined in `rdata.h`.

Merge branch 'each-rdatatype-functions' into 'main'

See merge request isc-projects/bind9!10216