Preconfirmed block p2p network: add scaffolding

[CHr15F0x]

Add support for the third p2p network in pathfinder. Right now the exact protocol is not specified except for the fact that gossipsub will be used. This is sufficient information to do the following:

• add new_preconfirmed() analogous to new_sync() and new_consensus() in the p2p crate; this implies adding a new app specific behavior that contains gossipsub,
• add p2p config for the new network in pathfinder
• add the new p2p network initialization code in pathfinder::src::p2p_network
• wire the initialization code in node_main
• design how the preconfimed block is passed from the new network entity to the RPC server

TBD: whether the preconfirmed block will be avalilable both through a REST API (as now) and the p2p network at the same time