arti-rpcserver: minimal handling for request.meta.require

We have defined a forward-compatibility mechanism for the RPC system, where a request may list one or more required "features", and if any feature is absent, the request will fail.

Since no "features" are currently implemented, this code does the bare minimum to implement this mechanism, by rejecting every request with a nonempty "require" field.

Part of #1662 (closed).

Remaining work will be to get the error.data field working again, but that is a bit heavier.