pageserver: handle temp_download files in secondary locations

[jcsp]

Problem

PR #6837 fixed secondary locations to avoid spamming log warnings on temp files, but we also have ".temp_download" files to consider.

Summary of changes

• Give temp_download files the same behavior as temp files.
• Refactor the relevant helper to pub(crate) from pub

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]

2472 tests run: 2349 passed, 0 failed, 123 skipped (full report)

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Flaky tests (1)

Postgres 16

• test_timeline_size_quota_on_startup: release

Code coverage* (full report)

• functions: 28.8% (6930 of 24085 functions)
• lines: 47.4% (42552 of 89856 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
93079c2 at 2024-03-01T15:42:27.415Z :recycle:}