Allow registering custom request-response protocols

[tomaka]

Just like one can register notifications protocols, one should be able to register targeted request-response protocols.

I'm opening this issue to keep track of that feature.

cc'ing @mxinden @rphmeier as we discussed this in a meeting a couple of weeks ago (when the world was not on lockdown)

[tomaka]

For what it's worth, this issue is low-priority to me, but if there's an urgent need I can probably open a properly-tested PR in a day or two.

[rphmeier]

We haven't had issues so far emulating request/response over notifications-based protocols, so I also don't view this as high-priority.

[tomaka]

Can now be done after #6559
The major obstacle to this now is to design an API.

[tomaka]

I briefly wanted to highlight that request-response protocols would show up in Grafana similar to this:

(one section like that per protocol)

Which is going to be very useful to figure out if things are working as expected.

[tomaka]

In #6634, starting a request to a node we're not connected to immediately fails. However, after chatting with Rob, there is a need for a function that tries to dial the node if we happen to not be connected to it.

[tomaka]

Reopening to address this comment:

In #6634, starting a request to a node we're not connected to immediately fails. However, after chatting with Rob, there is a need for a function that tries to dial the node if we happen to not be connected to it.

[tomaka]

Note-keeping: the reason for being able to perform requests to nodes we're not connected to is because we are not necessarily guaranteed a peer slot on the target node.

If, instead, there exists a system that guarantees a peer slot, then this isn't needed.