Provisioning: Fix bug when provision app plugins using Enterprise edition (#26340)

In OSS provisioning service init after plugin registration, but in
Enterprise it's the opposite order and installed app plugin check
fails. This adjusts service registry init priority to make sure plugins
are registered before provisioning inits.

Which issue(s) this PR fixes:
Fixes #26336

pkg/plugins/backendplugin/manager.go

@@ -34,11 +34,7 @@ var (
 )
 
 func init() {
-	registry.Register(&registry.Descriptor{
-		Name:         "BackendPluginManager",
-		Instance:     &manager{},
-		InitPriority: registry.Low,
-	})
+	registry.RegisterService(&manager{})
 }
 
 // Manager manages backend plugins.