I think it's time to introduce the notion of multiple different networks in the substrate-network crate.
We can already connect to multiple networks by creating multiple NetworkWorkers, but that's not a great solution as we duplicate a lot of resources, and is a problem if we ever switch to a single global DHT shared between all chains.

In terms of API, we should:

• Create a NetworkId struct that is an opaque struct around a u64. They don't have any meaning by themselves. We can decide that 0 means "relay chain", and N means "parachain N", but that's not the concern of substrate-network.
• Modify the config::Params struct to be able to pass multiple roles, Clients, specializations, etc. and associate them to a NetworkId.
• The on_block_imported, on_block_finalized, trigger_repropagate, announce_block, gossip_consensus_message, request_justification, and with_spec functions would get a NetworkId parameter to indicate which client we're talking about.

cc @bkchr