[network-data] add new DNS/SRP anycast/unicast service definitions (#6501)

This commit adds new network data service entry definitions to
indicate presence of DNS/SRP servers within the Thread mesh and
provide info about them. Two service TLV formats are added:

`NetworkData::Service::DnsSrpAnycast` indicates that DNS/SRP client on
a device can use the associated anycast address with this service TLV
to reach the DNS/SRP server. The use of anycast address ensures that
the messages are routed to the nearest node which has added such a
service entry in the network data. The service TLV data in this model
contains a one byte sequence number which is used to notify the SRP
clients if they need to re-register with the server(s) (e.g., due
server reset/reboot and/or loss of previous registrations).

`NetworkData::Service::DnsSrpUnicast` directly provides the IPv6
address and port info for a DNS/SRP server which can be included as
part of the service TLV data and/or the server TLV data. Using service
TLV data allows the info about a common infrastructure SRP/DNS server
to be added by multiple BRs. In the case of server TLV, the IPv6
address info can be optionally omitted (i.e. just a port number is
provided) which then causes the associated RLOC/ALOC address of the
node who added the service entry to be used as the server's IPv6
address.

This commit updates the `Srp::Client` to use the new service entries
to discover/select the SRP sever when auto-start mode is enabled. The
client prefers and uses a `DnsSrpAnycast` over `DnsSrpUnicast`
entries. The `Srp::Server` is also updated to publish its info as
`DnsSrpAnycast` entry (using mesh-local address).

This commit also updates the `test_network_data` unit test to cover
the behavior of newly methods which iterate and parse different
service entry formats.

tests/scripts/thread-cert/node.py

@@ -931,8 +931,9 @@ class NodeImpl:
             # TODO: for now, we are using 0xfd as the SRP service data.
             #       May use a dedicated bit flag for SRP server.
             if int(service[1], 16) == 0x5d:
-                # The SRP server data are 2-bytes UDP port number.
-                return int(service[2], 16)
+                # The SRP server data contains IPv6 address (16 bytes)
+                # followed by UDP port number.
+                return int(service[2][2 * 16:], 16)
 
     def srp_client_start(self, server_address, server_port):
         self.send_command(f'srp client start {server_address} {server_port}')