protofsm: add ability for state machine to consume wire msgs

In this commit, we add the ability for the state machine to consume wire
messages. This'll allow the creation of a new generic message router
that takes the place of the current peer `readHandler` in an upcoming
commit.

protofsm/msg_mapper.go

@@ -0,0 +1,15 @@
+package protofsm
+
+import (
+	"github.com/lightningnetwork/lnd/fn"
+	"github.com/lightningnetwork/lnd/lnwire"
+)
+
+// MsgMapper is used to map incoming wire messages into a FSM event. This is
+// useful to decouple the translation of an outside or wire message into an
+// event type that can be understood by the FSM.
+type MsgMapper[Event any] interface {
+	// MapMsg maps a wire message into a FSM event. If the message is not
+	// mappable, then an None is returned.
+	MapMsg(msg lnwire.Message) fn.Option[Event]
+}