[DNM] add filter iterator to disk btree.

[yliang412]

Problem

Summary of changes

Checklist before requesting a review

• I have performed a self-review of my code.
• If it is a core feature, I have added thorough tests.
• Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
• If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.

Checklist before merging

• Do not forget to reformat commit message to not include the above checklist

[github-actions]

5013 tests run: 4855 passed, 0 failed, 158 skipped (full report)

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Flaky tests (6)

Postgres 17

• test_pageserver_compaction_smoke: release-arm64

Postgres 16

• test_slots_and_branching: release-x86-64
• test_subscriber_restart: release-x86-64

Postgres 15

• test_subscriber_restart: release-x86-64

Postgres 14

• test_hot_standby_feedback: release-x86-64
• test_delete_timeline_client_hangup: release-x86-64

Code coverage* (full report)

• functions: 32.0% (7512 of 23445 functions)
• lines: 50.0% (60625 of 121150 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
5a87afd at 2024-09-27T14:57:50.365Z :recycle:}

[koivunej]

This seems to me like this could be a futures::future::Stream, it would give you the (async) map and sync filter(?) right away without writing these up.

[koivunej]

I never understood why was this boxed, it does not need to be.