Explore supporting client mode in dht nodes

[jsantell]

Updating to libp2p 0.53 introduced DHT nodes adjusting their modes dynamically, where if a node does not have an external address, it switches to Mode::Client. This improves the network by culling non-accessible nodes.
Manually setting to Mode::Server mode prevents this dynamic switching.
We could explore this optimization in the future, but for now, we need to run on nightly rust, and some challenges with external addresses in tests.