[monitoring] Allow collecting DHCP lease information in DeviceData.data

[nemesifier]

Like #30, but with DHCP lease information.

Incoming DHCP lease data:

{
    'type': 'DeviceMonitoring',
    'dhcp_leases': [
        {
            'expiry': '1586943200',
            'mac_address': 'f2:f1:3e:56:d2:77',
            'ip_address': '192.168.66.196',
            'client_name': 'MyPhone1',
            'client_id': '01:20:f4:78:19:3b:38'
        },
        {
            'expiry': '1586943120',  # integer
            'mac_address': 'f2:f1:3e:56:d2:66',  # mac address
            'ip_address': '192.168.66.197',  # ipv4 or ipv6
            'client_name': '*',  # string
            'client_id': '01:00:00:00:00:00:04'  # string
        }
    }
}

The schema in openwisp_monitoring.device.schema will have to be updated accordingly.

Once the data is saved and validated correctly (a few tests will need to ensure bogus leases will be rejected) we will have to ensure the leases are shown in the device status tab:
https://github.com/openwisp/openwisp-monitoring/blob/master/openwisp_monitoring/device/templates/admin/config/device/change_form.html#L66-L124

If the dhcp_leases info is not available, do not mention this in the admin UI.

Once this is ready, openwisp/openwrt-openwisp-monitoring#2 will have to be implemented on the device side.