[net] SRP server implementation (#5986)

This PR includes the initial implementation of the SRP server
defined in https://tools.ietf.org/html/draft-ietf-dnssd-srp-07.

- SRP server Service TLV propagation: Includes only the 2-bytes
  dynamic listening port in the Server TLV.
- SRP request processing (parsing & validation)
- LEASE & KEY-LEASE management
- SIG(0) verification
- Interface for advertising proxy
- CLI commands for testing
- Support removing indivitual SRP service

script/check-size

@@ -127,6 +127,7 @@ size_nrf52840_version()
         "SLAAC=1"
         "SNTP_CLIENT=1"
         "SRP_CLIENT=1"
+        "SRP_SERVER=1"
         "TIME_SYNC=1"
         "UDP_FORWARD=1"
     )