show rpc calls to bitcoin-core

To understand what Specter is doing in greater detail, this would be useful to have in the log somewhere the call to bitcoin-core, with the result back.

Bitcoin-core can be set up with `debug=rpc` but this just shows the method being called, not the args/result variable.

Maybe this is already what @mflaxman has been doing with: https://github.com/mflaxman/shared-bitcoin-node but I'm creating this issue to keep track of it.