1. Add new key for Agent2

net.dns.get[<ip>,name,<type>,<timeout>,<count>,<protocol>,<flags>]

1. extend type to support
   1. DNSSEC records: DNSKEY, CDNSKEY, DS, CDS, RRSIG, NSEC, NSEC3, NSEC3PARAM
   2. other records, e.g. CAA, NAPTR, DNAME, SSHFP (define in the spec)
2. treat flags as a comma-separated list of the following:
   1. cdflag, nocdflag (default) - maps to CD flag
   2. rdflag (default), nordflag - maps to RD flag
   3. dnssec, nodnssec (default) - maps to DO flag
   4. nsid, nonsid (default) - maps to NSID option
   5. edns0 (default), noedns0 - maps to EDNS0 settings
      1. dnssec is not allowed if noedns0 is set
3. return JSON that may include
   1. response flags (e.g. AA, RA)
   2. response code (NOERROR, SERVFAIL, etc)
   3. response sections: question, answer, authority, additional
   4. nsid
   5. query time
   6. error state (e.g. if DNS is down)

2. Agent and Agent2 changes

2.1 add net.dns.perf[<ip>,name,<type>,<timeout>,<count>,<protocol>]
return 0 if DNS is down, query time in seconds (with fractions) otherwise
2.2 improve handling of PTR requests in net.dns, net.dns.record, net.dns.perf (see ZBXNEXT-3826)
support both forms: net.dns[,4.3.2.1.in-addr.arpa,PTR] and net.dns[,1.2.3.4,PTR], including IPv6