feat: support var in suggested questions

[le0zh]

Summary

Added variable support for OPENING QUESTIONS, allowing recommended questions in the application to be controlled via variables.

Screenshots

Checklist

Important

Please review the checklist below before submitting your pull request.

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

[crazywoola]

What about export the DSL and import the DSL, will those variables are added as well?