feat(core): add AppModule tag interface (#13607)

core/appmodule/module.go

@@ -0,0 +1,14 @@
+package appmodule
+
+import "cosmossdk.io/depinject"
+
+// AppModule is a tag interface for app module implementations to use as a basis
+// for extension interfaces. It provides no functionality itself, but is the
+// type that all valid app modules should provide so that they can be identified
+// by other modules (usually via depinject) as app modules.
+type AppModule interface {
+	depinject.OnePerModuleType
+
+	// IsAppModule is a dummy method to tag a struct as implementing an AppModule.
+	IsAppModule()
+}