[feature] Allow enabling ipv4 forwarding

[nemesifier]

I am opening this issue to collect feedback, ideas, suggestions, reports on how routing and forwarding is configured in different deployments, so we should be able to figure out some good default routing/forwarding policy to add to the role (should be possible to disable it).

Before jumping to implement it though, we'll leave it open for discussion.

[nemesifier]

I think as a first step we can add a variable which enables ipv4 forwarding, eg: openwisp2_wireguard_ipv4_forwarding.