membuffer: support iterator for ART

[you06]

ref pingcap/tidb#55287

This PR implements Iterator for ART.

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cfzjywxk, ekexium

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[LGTM Timeline notifier]

Timeline:

• 2024-09-24 03:52:51.459401225 +0000 UTC m=+1538041.199825156: :ballot_box_with_check: agreed by cfzjywxk.
• 2024-09-24 11:35:02.287246603 +0000 UTC m=+1565772.027670543: :ballot_box_with_check: agreed by ekexium.