Add a proper supported API for obtaining node info programmatically

[jsdw]

I'm wondering how hard it would be to add a proper "API" to telemetry to expose the sort of information requested in #479. The reason being that the data sent to the UI is deliberately fairly minimal/compact (what the UI needs and no more), and also is not an interface we promise to support/not break.

Also, for the benefit of our future selves, having a clear split between "this is what we are sending to populate the UI" and "this is the more nicely structured and actually supported interface for obtaining node information programmatically" would be really valuable.

This API might be some sort of HTTP RESTful thing, or it might be something like JSONRPC over a websocket/http channel.

Let's gather up some of the information that is desirable to access programmatically here, so we can come up with an API to provide it.

[jsdw]

I added the solution-engineer label, though note that this issue isn't as well fleshed out as some.

I think a good starting point here would be to create a prototype/experiment branch where there is eg a JSON-RPC API (use jsonrpsee's server for this) that exposes/streams certain data as it comes in.

Once we have this working we can evaluate it and iterate slowly from there.