chore(pageserver): vectored get target_keyspace directly accums

[skyzh]

Problem

follow up on #7904

Summary of changes

avoid a layer of indirection introduced by Vec<Range<Key>>

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]

3216 tests run: 3074 passed, 0 failed, 142 skipped (full report)

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Flaky tests (2)

Postgres 16

• test_ondemand_wal_download_in_replication_slot_funcs: release

Postgres 15

• test_pageserver_restarts_under_worload: release

Code coverage* (full report)

• functions: 31.5% (6635 of 21061 functions)
• lines: 48.6% (51632 of 106259 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
e78146f at 2024-06-13T21:40:25.799Z :recycle:}

[VladLazar]

looks good to me. thank you for doing this