feat: add gemini-exp-1114

[hjlarry]

Summary

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Close issue syntax: Fixes #<issue number> or Resolves #<issue number>, see documentation for more details.

doc: https://ai.google.dev/gemini-api/docs/models/experimental-models

An experimental model seems get a high score in some arena website.

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