actor: add fundamental interfaces and concrete Actor impl

In this commit, we add the actual Actor implementation. We define a
series of types and interfaces, that in concert, describe our actor. An
actor has some ID, a reference (used to send messages to it), and also a
set of defined messages that it'll accept.

An actor can be implemented using a simple function if it's stateless.
Otherwise, a struct can implement the Receive method, and handle its
internal message passing and state that way.

actor/func_actor.go

@@ -0,0 +1,45 @@
+package actor
+
+import (
+	"context"
+
+	"github.com/lightningnetwork/lnd/fn/v2"
+)
+
+// ActorFunc is a function type that represents an actor which functions purely
+// based on a simple function processor.
+type ActorFunc[M Message, R any] func(context.Context, M) fn.Result[R]
+
+// FunctionBehavior adapts a function to the ActorBehavior interface.
+type FunctionBehavior[M Message, R any] struct {
+	fn ActorFunc[M, R]
+}
+
+// NewFunctionBehavior creates a behavior from a function.
+func NewFunctionBehavior[M Message, R any](
+	fn ActorFunc[M, R]) *FunctionBehavior[M, R] {
+
+	return &FunctionBehavior[M, R]{fn: fn}
+}
+
+// Receive implements ActorBehavior interface for the function.
+//
+// TODO(roasbeef): just base it off the function direct instead?
+func (b *FunctionBehavior[M, R]) Receive(ctx context.Context,
+	msg M) fn.Result[R] {
+
+	return b.fn(ctx, msg)
+}
+
+// FunctionBehaviorFromSimple adapts a simpler function to the ActorBehavior
+// interface.
+func FunctionBehaviorFromSimple[M Message, R any](
+	sFunc func(M) (R, error)) *FunctionBehavior[M, R] {
+
+	return NewFunctionBehavior(
+		func(ctx context.Context, msg M) fn.Result[R] {
+			val, err := sFunc(msg)
+			return fn.NewResult(val, err)
+		},
+	)
+}