Provided a configuration API endpoint for plugin functionality.
This allows users to describe the configuration struct of their plugins using AVRO schema.
During plugin runtime, the plugin's configuration can be accessed via the API.

Added new API endpoints:
- `/plugins/:name/schema`
  To get plugins avro schema and i18n config in one json object.
- `/plugins/:name/config`
  To get or update plugin's own config

Changed API endpoints:
- `/plugins/install`
  Status code when succeeded change to `204`. It was `200` previously.
- `/plugins/:name/move`
  Status code when succeeded change to `204`. It was `200` previously.