We build an in-memory model of what the API should look like, which
will later be used to generate a variety of bindings. In this PR we
will use the model to build structs corresponding to the requests and
responses for the various methods.
The JSON-RPC schemas serve as ground-truth, however they are missing a
bit of context: methods, and the request-response matching (as well as
a higher level grouping we'll call a Service). I'm tempted to create a
new document that describes this behavior and we could even generate
the rather repetitive JSON schemas from that document. Furthermore
it'd allow us to add some required metadata such as grpc field
numbering once we generate those bindings.
Changelog-Added: JSON-RPC: A new `msggen` library allows easy generation of language bindings for the JSON-RPC from the JSON schemas
