ipn/{ipnauth,ipnlocal,ipnserver}: send the auth URL to the user who started interactive login

We add the ClientID() method to the ipnauth.Actor interface and updated ipnserver.actor to implement it.
This method returns a unique ID of the connected client if the actor represents one. It helps link a series
of interactions initiated by the client, such as when a notification needs to be sent back to a specific session,
rather than all active sessions, in response to a certain request.

We also add LocalBackend.WatchNotificationsAs and LocalBackend.StartLoginInteractiveAs methods,
which are like WatchNotifications and StartLoginInteractive but accept an additional parameter
specifying an ipnauth.Actor who initiates the operation. We store these actor identities in
watchSession.owner and LocalBackend.authActor, respectively,and implement LocalBackend.sendTo
and related helper methods to enable sending notifications to watchSessions associated with actors
(or, more broadly, identifiable recipients).

We then use the above to change who receives the BrowseToURL notifications:
 - For user-initiated, interactive logins, the notification is delivered only to the user who initiated the
   process. If the initiating actor represents a specific connected client, the URL notification is sent back
   to the same LocalAPI client that called StartLoginInteractive. Otherwise, the notification is sent to all
   clients connected as that user.
   Currently, we only differentiate between users on Windows, as it is inherently a multi-user OS.
 - In all other cases (e.g., node key expiration), we send the notification to all connected users.

Updates tailscale/corp#18342

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

ipn/ipnserver/actor.go

@@ -31,6 +31,7 @@ type actor struct {
 	logf logger.Logf
 	ci   *ipnauth.ConnIdentity
 
+	clientID      ipnauth.ClientID
 	isLocalSystem bool // whether the actor is the Windows' Local System identity.
 }
 
@@ -39,7 +40,22 @@ func newActor(logf logger.Logf, c net.Conn) (*actor, error) {
 	if err != nil {
 		return nil, err
 	}
-	return &actor{logf: logf, ci: ci, isLocalSystem: connIsLocalSystem(ci)}, nil
+	var clientID ipnauth.ClientID
+	if pid := ci.Pid(); pid != 0 {
+		// Derive [ipnauth.ClientID] from the PID of the connected client process.
+		// TODO(nickkhyl): This is transient and will be re-worked as we
+		// progress on tailscale/corp#18342. At minimum, we should use a 2-tuple
+		// (PID + StartTime) or a 3-tuple (PID + StartTime + UID) to identify
+		// the client process. This helps prevent security issues where a
+		// terminated client process's PID could be reused by a different
+		// process. This is not currently an issue as we allow only one user to
+		// connect anyway.
+		// Additionally, we should consider caching authentication results since
+		// operations like retrieving a username by SID might require network
+		// connectivity on domain-joined devices and/or be slow.
+		clientID = ipnauth.ClientIDFrom(pid)
+	}
+	return &actor{logf: logf, ci: ci, clientID: clientID, isLocalSystem: connIsLocalSystem(ci)}, nil
 }
 
 // IsLocalSystem implements [ipnauth.Actor].
@@ -61,6 +77,11 @@ func (a *actor) pid() int {
 	return a.ci.Pid()
 }
 
+// ClientID implements [ipnauth.Actor].
+func (a *actor) ClientID() (_ ipnauth.ClientID, ok bool) {
+	return a.clientID, a.clientID != ipnauth.NoClientID
+}
+
 // Username implements [ipnauth.Actor].
 func (a *actor) Username() (string, error) {
 	if a.ci == nil {