network: add DHCP server domain name option support (#39260)

Implements DHCP option 15 (Domain Name) for systemd-networkd's DHCP
server, allowing administrators to configure the DNS default domain that
clients should use.

This addresses the feature request in issue #37077, where users needed
to manually configure domain names using
SendOption=15:string:example.com as a workaround.

This adds two new configuration options to the [DHCPServer] section:
- EmitDomain= (boolean): whether to send domain name to clients
- Domain= (string): the domain name to send (e.g., "example.com")

Example configuration:
  [DHCPServer] EmitDomain=yes Domain=example.com

This eliminates the need for manual workarounds using
SendOption=15:string:...

Fixes #37077

src/network/networkd-network.c

@@ -755,6 +755,7 @@ static Network *network_free(Network *network) {
         free(network->dhcp_server_boot_server_name);
         free(network->dhcp_server_boot_filename);
         free(network->dhcp_server_timezone);
+        free(network->dhcp_server_domain);
         free(network->dhcp_server_uplink_name);
         for (sd_dhcp_lease_server_type_t t = 0; t < _SD_DHCP_LEASE_SERVER_TYPE_MAX; t++)
                 free(network->dhcp_server_emit[t].addresses);