feat(pageserver): support split delta layers

[skyzh]

Problem

part of #8002

Summary of changes

Similar to #8574, we add auto-split support for delta layers. Tests are reused from image layer split writers.

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]

2134 tests run: 2065 passed, 0 failed, 69 skipped (full report)

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Flaky tests (1)

Postgres 15

• test_sql_over_http2: release

Code coverage* (full report)

• functions: 32.9% (7155 of 21726 functions)
• lines: 50.6% (57764 of 114231 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
8e89f16 at 2024-08-05T10:38:19.824Z :recycle:}