ipn/ipn{auth,server}: update ipnauth.Actor to carry a context

The context carries additional information about the actor, such as the
request reason, and is canceled when the actor is done.

Additionally, we implement three new ipn.Actor types that wrap other actors
to modify their behavior:
 - WithRequestReason, which adds a request reason to the actor;
 - WithoutClose, which narrows the actor's interface to prevent it from being
   closed;
 - WithPolicyChecks, which adds policy checks to the actor's CheckProfileAccess
   method.

Updates #14823

Signed-off-by: Nick Khyl <nickk@tailscale.com>

ipn/ipnauth/test_actor.go

@@ -4,6 +4,8 @@
 package ipnauth
 
 import (
+	"cmp"
+	"context"
 	"errors"
 
 	"tailscale.com/ipn"
@@ -17,6 +19,7 @@ type TestActor struct {
 	Name        string            // username associated with the actor, or ""
 	NameErr     error             // error to be returned by [TestActor.Username]
 	CID         ClientID          // non-zero if the actor represents a connected LocalAPI client
+	Ctx         context.Context   // context associated with the actor
 	LocalSystem bool              // whether the actor represents the special Local System account on Windows
 	LocalAdmin  bool              // whether the actor has local admin access
 }
@@ -30,6 +33,9 @@ func (a *TestActor) Username() (string, error) { return a.Name, a.NameErr }
 // ClientID implements [Actor].
 func (a *TestActor) ClientID() (_ ClientID, ok bool) { return a.CID, a.CID != NoClientID }
 
+// Context implements [Actor].
+func (a *TestActor) Context() context.Context { return cmp.Or(a.Ctx, context.Background()) }
+
 // CheckProfileAccess implements [Actor].
 func (a *TestActor) CheckProfileAccess(profile ipn.LoginProfileView, _ ProfileAccess, _ AuditLogFunc) error {
 	return errors.New("profile access denied")