Request: Add support for additional OPT interface

[lars18th]

Hi @AlbrechtL ,

I have a request to support a third network interface. But before any discussion I prefer to comment on the different potential use cases for this new interface:

• Third interface for a GUEST network.
• Or third interface for a out-of-band ADMIN network.
• Or third interface for WAN2 connection.
• Or third interface for a IoT isolated interface.
• etc.

Now it's possible to configure, for example, a tunnel or a QinQ trunk to provide multiple interfaces for the OpenWRT instance. However, this is quite complex to configure using a Docker container. And it's necessary some external support. Therefore, my request is to provide a more simple solution: support for a third (an optional) OPT interface (disabled by default).

Do you think it might be difficult to add it, and does it make sense from your point of view?
Please consider this as it will be very useful to me in the use case where I want to deploy it.
Thank you.