Support for "real" Ethernet hardware

[kripton]

The current implementation emulates a network device via USB (CDC NCM) so that the host computer can transfer data via IP packets (web interface, ArtNet, E1.31/sACN). However, attaching a real Ethernet device could be beneficial. One could build a standalone 16-port ArtNet-to-DMX adapter.

However, the rp2040 only has a limited amount of GPIO pins and there are probably not enough to cater for a LAN8720 board (except if some of its pins were optional). There might be Ethernet-Boards that attach via I2C (no additional pins required) or SPI (one additional pin required for the CE line).

This will also require quite some refactoring on the software side. We could go all the way to allow packet forwarding from the emulated USB network interface to the "real" Ethernet hardware, so the dongle could be used as USB-to-Ethernet adapter as well. I assume that would not be worth the effort ;)