chore(pageserver): add force aux file policy switch handler

[skyzh]

Problem

For existing users, we want to allow doing a force switch for their aux file policy.

Part of #7462

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

[github-actions]

3108 tests run: 2982 passed, 0 failed, 126 skipped (full report)

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Flaky tests (4)

Postgres 16

• test_timeline_deletion_with_files_stuck_in_upload_queue: debug
• test_synthetic_size_while_deleting: debug

Postgres 15

• test_synthetic_size_while_deleting: debug
• test_vm_bit_clear_on_heap_lock: debug

Code coverage* (full report)

• functions: 31.4% (6451 of 20515 functions)
• lines: 48.3% (49865 of 103237 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
d6cb086 at 2024-05-22T19:13:15.446Z :recycle:}