DNS : recurse for ptr records only if recursion is desired and configured

[the-nando]

Description

This PR checks that the DNS request for the PTR has rd set (RecursionDesired) and we have recursors configured before calling the handleRecurse function.

Fixes:

Testing & Reproduction steps

Issue with calling handleRecurse if not neeed:

• Start Consul without recursors configured
• dig -p 8600 -x 127.0.0.3 @localhost
• It should return NXDOMAIN but returns SERVFAIL

Issue with lack of handling of the rd flag:

• Start Consul with recursors = ["1.1.1.1"] configured
• dig -p 8600-x 1.1.1.1 @localhost +norecurse
• It should return NXDOMAIN but returns one.one.one.one.

Links

Fixes: https://github.com/hashicorp/consul/issues/20413

PR Checklist

• [ ] updated test coverage
• [x] external facing docs updated
• [x] appropriate backport labels added
• [x] not a security concern