Implement option_simple_close as defined in the official spec feature in lightning/bolts#1205

Hopefully this is the last time we change the mutual close protocol! And at some point that will let us entirely remove all the code supporting the two previous mutual close protocols (this is why I kept the code as separate as possible instead of trying to fit into the existing NEGOTIATING state).

Note that this is a prerequisite for taproot channels: this protocol allows nodes to safely exchange nonces in shutdown, closing_complete and closing_sig to spend a musig2 channel output.

I have verified that we have cross-compat with lnd so this is ready to integrate!