Add support for custom network topologies

[ema-pe]

Currently the DFaaS nodes are connected in a peer-to-peer network, where each node is directly connected to all others. It may be interesting to explore ways to define custom network topologies where each node is connected to all the others ("control plane"), but requests are forwarded only to some nodes called "neighbors". These neighbors can be discovered based on measurable parameters, such as latency (see #6), or they can be statically defined. There is an issue from libp2p's repository (libp2p/notes/issues/13) where custom topologies are discussed.

[aletundo]

As mentioned the other day in our pvt chat, you might also consider multiple pub-sub topics as a workaround/alternative to "actual" custom network topologies (see https://docs.libp2p.io/concepts/pubsub/overview/).