client/local,cmd/tailscale/cli,ipn/localapi: expose eventbus graph (#16597)

Make it possible to dump the eventbus graph as JSON or DOT to both debug
and document what is communicated via the bus.

Updates #15160

Signed-off-by: Claus Lensbøl <claus@tailscale.com>

util/eventbus/debug.go

@@ -195,3 +195,16 @@ type DebugEvent struct {
 	To    []string
 	Event any
 }
+
+// DebugTopics provides the JSON encoding as a wrapper for a collection of [DebugTopic].
+type DebugTopics struct {
+	Topics []DebugTopic
+}
+
+// DebugTopic provides the JSON encoding of publishers and subscribers for a
+// given topic.
+type DebugTopic struct {
+	Name        string
+	Publisher   string
+	Subscribers []string
+}