Allow custom protocols to participate in RPC

[rklaehn]

Custom protocols are currently second class citizens because they can't participate in RPC. We have already done some steps to modularize RPC in quic-rpc and elsewhere.

Not sure if this can be done without making the default case more unwieldy, but ideally custom protocols should get the option to participate in RPC. Basically each custom protocol handler would also get a look at each RPC message and have the option to handle it.

With this we could thin down the giant switch statement that is our current RPC handler, so that only really node internal RPCs would be handled directly by the node.