Feature: Guarantee params as objects when doing the plugin RPC passthrough

[fiatjaf]

Today it is painful to write plugins because we never know if the user will send the parameters as an array or as objects, so we have to handle both.

Life would be much better if lightningd parsed the params and turned them into an object -- forllowing the pattern specified in the usage string -- then only passed them as objects for the plugins.

Thank you!