Tracking issue of GSP-837: Support Feature Flag

[JinnyYi]

• Rendered GSP

Steps

• [x] Add operations back to Storager
  • Test should not use type assertions.
• [x] Support feature flag API
  • Add write_empty_object in features.toml and add virtual property for virtual features.
  • Generate ServiceFeatures and StorageFeatures on go-storage side. Rename the current generated ServiceFeatures and StorageFeatures pairs and structs for services.
• [ ] Integration test supports feature flag
  • tests: Migrate all tests to Feature and suite based
• [x] Services migrate to go-storage
  • Update dot-import for the types package