client/web: add exit node selector

Add exit node selector (in full management client only) that allows
for advertising as an exit node, or selecting another exit node on
the Tailnet for use.

Updates #10261

Signed-off-by: Sonia Appasamy <sonia@tailscale.com>

client/web/web.go

@@ -528,6 +528,9 @@ func (s *Server) serveAPI(w http.ResponseWriter, r *http.Request) {
 			http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
 		}
 		return
+	case path == "/exit-nodes" && r.Method == httpm.GET:
+		s.serveGetExitNodes(w, r)
+		return
 	case strings.HasPrefix(path, "/local/"):
 		s.proxyRequestToLocalAPI(w, r)
 		return
@@ -560,6 +563,7 @@ type nodeData struct {
 	UnraidToken string
 	URLPrefix   string // if set, the URL prefix the client is served behind
 
+	ExitNodeStatus    *exitNodeWithStatus
 	AdvertiseExitNode bool
 	AdvertiseRoutes   string
 	RunningSSHServer  bool
@@ -634,9 +638,62 @@ func (s *Server) serveGetNodeData(w http.ResponseWriter, r *http.Request) {
 			data.AdvertiseRoutes += r.String()
 		}
 	}
+	if e := st.ExitNodeStatus; e != nil {
+		data.ExitNodeStatus = &exitNodeWithStatus{
+			exitNode: exitNode{ID: e.ID},
+			Online:   e.Online,
+		}
+		for _, ps := range st.Peer {
+			if ps.ID == e.ID {
+				data.ExitNodeStatus.Name = ps.DNSName
+				data.ExitNodeStatus.Location = ps.Location
+				break
+			}
+		}
+		if data.ExitNodeStatus.Name == "" {
+			// Falling back to TailscaleIP/StableNodeID when the peer
+			// is no longer included in status.
+			if len(e.TailscaleIPs) > 0 {
+				data.ExitNodeStatus.Name = e.TailscaleIPs[0].Addr().String()
+			} else {
+				data.ExitNodeStatus.Name = string(e.ID)
+			}
+		}
+	}
 	writeJSON(w, *data)
 }
 
+type exitNode struct {
+	ID       tailcfg.StableNodeID
+	Name     string
+	Location *tailcfg.Location
+}
+
+type exitNodeWithStatus struct {
+	exitNode
+	Online bool
+}
+
+func (s *Server) serveGetExitNodes(w http.ResponseWriter, r *http.Request) {
+	st, err := s.lc.Status(r.Context())
+	if err != nil {
+		http.Error(w, err.Error(), http.StatusInternalServerError)
+		return
+	}
+	var exitNodes []*exitNode
+	for _, ps := range st.Peer {
+		if !ps.ExitNodeOption {
+			continue
+		}
+		exitNodes = append(exitNodes, &exitNode{
+			ID:       ps.ID,
+			Name:     ps.DNSName,
+			Location: ps.Location,
+		})
+	}
+	writeJSON(w, exitNodes)
+}
+
 type nodeUpdate struct {
 	AdvertiseRoutes   string
 	AdvertiseExitNode bool