Multicast DNS Discovery Driver
------------------------------
An OPC UA application can announce / retract a server via multicast DNS. Also
it receives multicast-based information about other servers. This
functionality is implemented as a driver that integrates with open62541
servers.
Internally, the drivers use the EventLoop of the server to create and manage
UDP sockets. They further use the UA_APPLICATIONNOTIFICATIONTYPE_DISCOVERY
category for server notifications to receive relevant updates from the server
at runtime.

The following configuration parameters are forwarded in a key-value map
during driver instantiation and/or set in the key-value map inside the
generic driver structure before starting the driver.

0:listen [bool]
   Receive information about OPC UA servers on the network via mDNS. Calls
   UA_Server_registerServerOnNetwork and UA_Server_deregisterServerOnNetwork
   to store the server information locally.

0:announce [bool]
   Announce registered servers over mDNS. This receives register/deregister
   notifications from UA_Server_registerServerOnNetwork and forwards the
   information.

0:announce-ttl [uint32]
   Default TTL in seconds for announced mDNS records.

0:query-presence [bool]
   Send an mDNS PTR query for _opcua-tcp._tcp.local. on startup. Defaults to
   false.

0:query-details [bool]
   Send targeted SRV/TXT follow-up queries for partially received OPC UA
   service instances. Defaults to false.

0:query-interval [uint32]
   Interval in seconds for repeated presence queries. A value of zero
   disables repeated queries. Defaults to zero.

0:interface [string]
   Network interface for listening or sending. Can be either the IP address
   of the network interface or the interface name (e.g. 'eth0'). Cf. the UDP
   ConnectionManager parameters.

.. code-block:: c

   
   typedef struct UA_MdnsDriver UA_MdnsDriver;
   
   struct UA_MdnsDriver {
       UA_Driver drv; /* Must be the first member */
   
       /* Manually publish/remove address records. This is separate from the
        * ServerOnNetwork announcement path as PTR/SRV/TXT records are always
        * derived from the ServerOnNetwork data. The address strings are numeric
        * IPv4/IPv6 literals. */
       UA_StatusCode (*addARecord)(UA_MdnsDriver *mdns, UA_String hostname,
                                   UA_String address, UA_UInt32 ttl);
       UA_StatusCode (*removeARecord)(UA_MdnsDriver *mdns, UA_String hostname,
                                      UA_String address);
       UA_StatusCode (*addAAAARecord)(UA_MdnsDriver *mdns, UA_String hostname,
                                      UA_String address, UA_UInt32 ttl);
       UA_StatusCode (*removeAAAARecord)(UA_MdnsDriver *mdns, UA_String hostname,
                                         UA_String address);
   };
   
   /* Based on the libmdnsd dependency */
   UA_MdnsDriver *
   UA_MdnsDriver_Mdnsd(const UA_KeyValueMap params);
   
   /* Based on the libavahi dependency */
   UA_MdnsDriver *
   UA_MdnsDriver_Avahi(const UA_KeyValueMap params);