feat: answer direct queries for nameservers

[agaffney]

Fixes #62

[agaffney]

For test domain cardano. with nameserver entry ns1.cardano (172.28.0.2)

$ dig @localhost -p 8053 ns1.cardano

; <<>> DiG 9.18.12-0ubuntu0.22.04.3-Ubuntu <<>> @localhost -p 8053 ns1.cardano
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 36756
;; flags: qr rd; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0
;; WARNING: recursion requested but not available

;; QUESTION SECTION:
;ns1.cardano.                   IN      A

;; ANSWER SECTION:
ns1.cardano.            999     IN      A       172.28.0.2

;; Query time: 0 msec
;; SERVER: 127.0.0.1#8053(localhost) (UDP)
;; WHEN: Thu Oct 26 15:11:01 CDT 2023
;; MSG SIZE  rcvd: 56

$ dig @localhost -p 8053 ns2.cardano

; <<>> DiG 9.18.12-0ubuntu0.22.04.3-Ubuntu <<>> @localhost -p 8053 ns2.cardano
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 16557
;; flags: qr rd; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 1
;; WARNING: recursion requested but not available

;; QUESTION SECTION:
;ns2.cardano.                   IN      A

;; AUTHORITY SECTION:
cardano.                999     IN      NS      ns1.cardano.

;; ADDITIONAL SECTION:
ns1.cardano.            999     IN      A       172.28.0.2

;; Query time: 0 msec
;; SERVER: 127.0.0.1#8053(localhost) (UDP)
;; WHEN: Thu Oct 26 15:11:07 CDT 2023
;; MSG SIZE  rcvd: 88