feat(pageserver): support partial gc-compaction for lowest retain lsn

[skyzh]

Problem

part of #8921, #9114

Summary of changes

We start the partial compaction implementation with the image layer partial generation. The partial compaction API now takes a key range. We will only generate images for that key range for now, and remove layers fully included in the key range after compaction.

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]

5292 tests run: 5070 passed, 0 failed, 222 skipped (full report)

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Code coverage* (full report)

• functions: 31.4% (7748 of 24639 functions)
• lines: 49.0% (60989 of 124526 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
4aa8ec6 at 2024-10-29T17:30:44.815Z :recycle:}