rpc: pass "params" keyword dictionary to clightning

[SimonVrouwe]

Together with https://github.com/shesek/lightning-client-js/pull/4 this passes "params" {key: val, ...} dictionary directly to c-lightning. So we can use keywords instead of exactly ordered (and obligatory null) positional args.

A "params" [ val, val2,... ] array with positional args is still accepted, so I think clients can migrate slowly.

Hope this doesn't break anything, a basic regtest with existing Spark-client v0.3.2-rc (on emulator) showed no issues. Inspired while trying to fix the Spark/Sparko backend of Zeus wallet.

edit: Spark's custom rpc methods, _listinvoices, _listpays etc. still require positional args to be passes as array.