Add RFC 2782 SRV RR Type Support

Implement encoding and decoding for the SRV resource record type (TYPE=33) specified in RFC 2782.

• Allows magna to parse the structured RDATA (uint16 Priority, uint16 Weight, uint16 Port, Target Name) used for service discovery.
• Allows magna to generate SRV records from their constituent parts, correctly handling domain name encoding for the Target Name.