feat(pageserver): support inherited sparse keyspace

[skyzh]

Problem

In preparation to #9516. We need to store rel size and directory data in the sparse keyspace, but it does not support inheritance yet.

Summary of changes

Add a new type of keyspace "sparse but inherited" into the system.

On the read path: we don't remove the key range when we descend into the ancestor. The search will stop when (1) the full key range is covered by image layers (which has already been implemented before), or (2) we reach the end of the ancestor chain.

[skyzh]

handling image layers seems a little bit tricker, I'll need to revisit and add more tests

[github-actions]

7271 tests run: 6907 passed, 0 failed, 364 skipped (full report)

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Flaky tests (1)

Postgres 16

• test_physical_replication_config_mismatch_max_locks_per_transaction: release-arm64

Code coverage* (full report)

• functions: 32.5% (8042 of 24739 functions)
• lines: 47.8% (66915 of 139938 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
c857660 at 2025-01-09T21:19:32.692Z :recycle:}

[skyzh]

ready for review :)