Expose usable API

[jsdw]

People like the 1kv program need to be able to programmatically access data about nodes. Currently, they do this via the "feed" data that's sent to the telemetry UI to populate it. This feed interface sucks; it's not supposed to be a "public" interface and we don't want to commit to it being stable. Much better would be to programmatically expose specific data that's of interest (via some sort of subscriptions) in a more accessible and easy to work with format. This is then a format we can endavour to maintain some level of support for and clearly signal any breaking changes to.

It's come up a number of times where people have requested such an interface. See eg #562 more recently. @ggwpez was also asking about extracting validator version info.