feat(pageserver): add metadata key range and aux key encoding

[skyzh]

Problem

Extracted from #7375. We assume everything >= 0x80 are metadata keys. AUX file keys are part of the metadata keys, and we use 0x90 as the prefix for AUX file keys.

The AUX file encoding is described in the code comment. We use xxhash128 as the hash algorithm. It seems to be portable according to the introduction,

xxHash is an Extremely fast Hash algorithm, processing at RAM speed limits. Code is highly portable, and produces hashes identical across all platforms (little / big endian).

...though whether the Rust version follows the same convention is unknown and might need manual review of the library. Anyways, we can always change the hash algorithm before rolling it out in staging/end-user, and I made a quick decision to use xxhash here because it generates 128b hash + portable. We can save the discussion of which hash algorithm to use later.

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

[skyzh]

I'm thinking about adding some test cases for key ordering, maybe in later pull requests. Ordering of keys are not important as we are using hashing for AUX keys. It might be important if at some point we transform to variable-sized keys or have some metadata keys requiring ordering when scanning.

[github-actions]

2766 tests run: 2646 passed, 0 failed, 120 skipped (full report)

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Flaky tests (1)

Postgres 16

• test_vm_bit_clear_on_heap_lock: debug

Code coverage* (full report)

• functions: 28.1% (6482 of 23065 functions)
• lines: 46.9% (45932 of 97884 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
e48f8d9 at 2024-04-23T15:23:06.848Z :recycle:}

[skyzh]

checked by default non-rel-block key is on shard 0 as in key_is_shard0

[skyzh]

reserving merge conflict with #7404 and not adding aux key range into is_inherited_keys. need to fix it in the future.