Add config option for disabling RPC

Currently, when the experimental feature is enabled, arti will automatically listen on the configured rpc_listen unix domain socket. It would be nice to have the option to disable rpc altogether (for example !2272 (34171221)).

(Because this behavior currently can't be disabled through the TOML config, when compiling arti for the shadow tests, we avoid passing --features experimental, because that enables rpc and shadow doesn't support pathname-bound unix domain sockets)

Note: I originally added an enabled config option in !2272 (merged) (see !2272 (comment 3049727)), but ended up reverting the change: !2272 (comment 3056283)).

cc @nickm