[REQUIREMENT][Singleton Service] Support storing comment for resource string

Is your feature request related to a problem? Please describe.
When collecting resource strings to Singleton Service, a "comment" field is supported so that users can add some explanation information to the translatable resource string. However, Singleton Service directly forwards the comments to other translation service in the downstream, without storing them in the translation bundle.

With this design, it's not possible for community users to get the comment information if they deploy Singleton Service independently, not connecting to any other translation service.

Describe the solution you'd like
Store the comment information when storing resource strings in translation bundle files (messages_xx.json)

Describe alternatives you've considered
N/A

Additional context
N/A

Start by locating the Singleton Service code that stores resource strings in translation bundle files such as messages_xx.json. Trace how the comment field is handled and define the bundle representation needed so comments remain available when no downstream translation service is connected; verify the stored bundle preserves the comment.