Use only one network service for all chains

[tomaka]

paritytech/smoldot#997

[tomaka]

While this would be a nice to have feature, it is unfortunately insanely complicated.

[tomaka]

The plan, to implement that, is to no longer pass the list of protocols to peers.rs or collection.rs (or anything below) ahead of time.

Instead, the parameters of the protocol (including its name) are now passed every single time a substream is opened. When the remote opens a substream, it generates an event which the service.rs must answer, to determine the type and parameters of the protocol requested by the remote.
For this reason, it is easier if #59 done beforehand.

This would make it possible for service.rs to support additional protocols and remove support for existing protocols. Then, the API of service.rs could be changed to allow adding or removing entire chains.

Note that when it comes to peers.rs, since the state machine must track duplicate substreams, it is unclear how exactly this would look like at the moment.

[tomaka]

Note that it is possible for multiple chains with the same genesis hash and fork ID to be added to the light client.
When that happens, all the chains with the same genesis hash and fork ID should use the same ChainId of that networking service.

This has the consequence that the network service must change to be able to report the existing state of the networking, which is something we already want to do anyway for #519.