[dnssd] implement DNS-SD Discovery Proxy (#6191)

This commit implements DNS-SD Discovery Proxy.
- Implemented DNS-SD Discovery Proxy functionalities - OT part
- Fixed an memory issue of NameCompressInfo (introduced in #6155 , but
  somehow revealed by this commit)

Some implementation details:
- Discovery Proxy subscribes to services/service instances/hosts
  through callbacks set by a public OT API. It is up to the platform
  mDNS implementation to collect service instance/host information and
  notify OT via a public OT API.
- Discovery Proxy can handle DNS browsing of one service or DNS
  resolving of one service instance/host. We leave browsing multiple
  services or resolving multiple service instances/hosts for future
  enhancements if necessary.

tests/scripts/thread-cert/config.py

@@ -144,6 +144,8 @@ LEADER_NOTIFY_SED_BY_CHILD_UPDATE_REQUEST = True
 THREAD_VERSION_1_1 = 2
 THREAD_VERSION_1_2 = 3
 
+BORDER_AGENT_UDP_PORT = 49191
+
 
 def create_default_network_data_prefix_sub_tlvs_factories():
     return {