feat(pageserver): add delta layer iterator

[skyzh]

Problem

part of #8002

Summary of changes

Add delta layer iterator and tests.

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]

2940 tests run: 2824 passed, 0 failed, 116 skipped (full report)

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Flaky tests (4)

Postgres 16

• test_lr_with_slow_safekeeper: release

Postgres 14

• test_subscriber_restart: release
• test_wal_restore: release
• test_wal_restore_initdb: release

Code coverage* (full report)

• functions: 32.7% (6913 of 21143 functions)
• lines: 50.1% (54092 of 108061 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
9a901a5 at 2024-06-27T16:12:04.743Z :recycle:}

[VladLazar]

Looks good to me