lnwallet: add new aux resolver interface

This will be used by external callers to modify the way we resolve
contracts on chain. For a given contract, we'll store an extra "blob",
that will later be presented during the sweeping phase.

server.go

@@ -1631,6 +1631,7 @@ func newServer(cfg *Config, listenAddrs []net.Addr,
 			br, err := lnwallet.NewBreachRetribution(
 				channel, commitHeight, 0, nil,
 				implCfg.AuxLeafStore,
+				implCfg.AuxContractResolver,
 			)
 			if err != nil {
 				return nil, 0, err