rgw: add headers to guide cache update in 304 response

[IlsooByun]

Fixes: https://tracker.ceph.com/issues/45736
Signed-off-by: Ilsoo Byun ilsoobyun@linecorp.com

Checklist

• References tracker ticket
• Updates documentation if necessary
• Includes tests for new functionality or reproducer for bug

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[stale]

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[stale]

This pull request has been automatically closed because there has been no activity for 90 days. Please feel free to reopen this pull request (or open a new one) if the proposed change is still appropriate. Thank you for your contribution!

[cbodley]

reopened in #54587