We could've added a storage API, and fetched fields that way, except we want
the storage to be opaque. That meant we needed to add the RPC routes to the
node, which also simplifies other people writing RPC code and fetching these
fields. Then the node could've used the storage API, except a lot of the
storage in validator-sets is marked opaque and to only be read via functions,
so extending the runtime made the most sense.
