Zimfarm worker: add support for multiple outgoing IPs

[benoit74]

Currently, Zimfarm worker does not manage at all the outgoing traffic, meaning it uses whatever default route is configured in the OS.

When a worker has multiple public IP, we would benefit from taking advantage of that by spreading the task on the various public IPs available (which would be exposed as various adapters at the host level).

Not sure how much effort it would necessitate, and I think that we've just encountered the first worker which make take benefit from this new functionality, might well be an edge case.