move plugin management from ui-core to server-core

Summary:
Follow up of D32665064, this diff moves all plugin management logic from flipper-ui to flipper-server. Things like downloading, installing, querying new plugins.

Loading plugins is handled separately in the next diff.

Reviewed By: nikoant

Differential Revision: D32666537

fbshipit-source-id: 9786b82987f00180bb26200e38735b334dc4d5c3

desktop/flipper-server-core/src/fb-stubs/constants.tsx

@@ -0,0 +1,10 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @format
+ */
+
+export const isFBBuild: boolean = false;