feat(slow_query): download db file for slow query

[elleryasimov]

Enable download slow query db file on slow query page in clinic-cloud.

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Welcome @elleryasimov! It looks like this is your first PR to pingcap/tidb-dashboard 🎉

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: baurine

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[LGTM Timeline notifier]

Timeline:

• 2024-04-19 02:57:51.243283918 +0000 UTC m=+566090.354330364: :ballot_box_with_check: agreed by baurine.