feat: support opening new tab in markdown button

[bowenliang123]

Summary

• support open new tab with URL in markdown button
• follow-up with feat: support button in markdown #9876

We need to define one new attribute data-link.

• data-link: controls the URL for new page

Note:

• NO breaking chages in frontend usage. Previously, markdown display supported opending URLs in <a href="..."></a>

• Example:

<button data-link="http://https://dify.ai/" data-variant="primary" data>Dify</button>

Screenshots

Before	After
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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

[bowenliang123]

cc @crazywoola