This issue is intended to provide an outline of the necessary steps to complete support for option_simple_close (cf. lightning/bolts#1205)

• Add the feature bit/message types/support for OP_RETURN in ShutdownScript (option_simple_close: Add feature, messaging types, shutdown script construction #3861 )
• Pre-/Refactor channel closing state
  • e.g., move signer_pending_closing/pending_counterparty_closing_signed to a state enum that can be reused with respective variants for ClosingComplete/ClosingSig
• Add handling logic for ClosingComplete/ClosingSig, adjust closing transaction construction
• Add support for RBF'ing closing transactions