extension: Add MessageStored event (#316)

* Replace existing direct StoreManager->msghub communication with this
  event
* For #280 #309 #312 #310

Signed-off-by: James Hillyerd <james@hillyerd.com>

Signed-off-by: James Hillyerd <james@hillyerd.com>

pkg/extension/host.go

@@ -0,0 +1,23 @@
+package extension
+
+import (
+	"github.com/inbucket/inbucket/pkg/extension/event"
+)
+
+// Host defines extension points for Inbucket.
+type Host struct {
+	Events *Events
+}
+
+// Events defines all the event types supported by the extension host.
+type Events struct {
+	MessageStored EventBroker[event.MessageMetadata, Void]
+}
+
+// Void indicates the event emitter will ignore any value returned by listeners.
+type Void struct{}
+
+// NewHost creates a new extension host.
+func NewHost() *Host {
+	return &Host{Events: &Events{}}
+}