Add possibility to set MTU for PPPoE
Some ISPs specify specific MTU. Without setting it the connection does not work. In case of DHCP this is pretty much non-existent but it is pretty common in DSL lands. It is pretty stupid as solution is rather "fix your network" but at the moment this and VLAN mentioned in https://gitlab.nic.cz/turris/project/-/issues/96 block simple usage on some ISPs networks. It is single UCI option and one numerical field (possibly can be hidden under some "advanced" roll up.