Add support for mDNS registration

[fadushin]

In order to interoperate with services like Home Assistant, it would be good if a device running AtomVM could register services it is running via mDNS.

Proposal:

Augment the current network interface to allow configuration of mDNS entries, when the device is configured in STA mode.

This change just requires the addition of an optional field in the STA configuration. If the mDNS configuration is not present, mDNS is not initialized. Otherwise, mDNS is initialized, as each entry in configuration is added to the internal mdns server. Note that this list may be empty, though at present there would be no use case for this. (In the future, we may want to add an API for mDNS query, but this issue only addresses registration).

Here is the suggested type specification:

-type sta_config_property() ::
    ssid_config()
    | psk_config()
    | dhcp_hostname_config()
    | mdns_config(),   %% <-- new
    | sta_connected_config()
    | sta_disconnected_config()
    | sta_got_ip_config().

-type mdns_config() :: {mdns, [mdns_entry()]}.
-type mdns_entry() ::
    [
        {service_name, string()}, %% optional
        {protocol, tcp | udp}, %% required
        {port, port_number()}, %% required
        {txt_data, [{string(), string()}]} %% optional
    ].

Note that this proposal does not support modification to mDNS entries after initialization. A future API could be crafted to support this.

[UncleGrumpy]

This would be a great improvement! We should make it an option in AP mode as well, in case the user wants to start a private network, but be able to reach the host via mdns.

I think eventually we might want to break out a separate module to add more mdns features, in addition to query like adding or removing services and delegate hosts after the network has been started.