[Network] /v3/diagnostics endpoint

[jcnelson]

We spend a lot of time trawling logs for the same kinds of data. Instead, the node could just make the data available in a machine-readable form (e.g. JSON) over an RPC endpoint (proposing /v3/diagnostics). The API doesn't need to be stable, but it should provide a view into the node's state that's relevant to debugging common issues.