pageserver: move page_service's import basebackup / import wal to mgmt API

[problame]

I want to fix bugs in page_service (issue) and the import basebackup / import wal stand in the way / make the refactoring more complicated.

We don't use these methods anyway in practice, but, there have been some objections to removing the functionality completely.

So, this PR preserves the existing functionality but moves it into the HTTP management API.

Note that I don't try to fix existing bugs in the code, specifically not fixing

• it only ever worked correctly for unsharded tenants
• it doesn't clean up on error

All errors are mapped to ApiError::InternalServerError.

[github-actions]

3042 tests run: 2927 passed, 0 failed, 115 skipped (full report)

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Flaky tests (5)

Postgres 16

• test_ondemand_wal_download_in_replication_slot_funcs: release

Postgres 15

• test_isolation[None]: debug

Postgres 14

• test_background_operation_cancellation: release
• test_subscriber_restart: release
• test_tenant_creation_fails: debug

Code coverage* (full report)

• functions: 32.6% (6931 of 21277 functions)
• lines: 50.0% (54483 of 109016 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
21ebb69 at 2024-07-05T20:56:28.874Z :recycle:}

[yliang412]

LGTM

[problame]

Waiting for review from team compute https://neondb.slack.com/archives/C04DGM6SMTM/p1720542945296569

[hlinnaka]

Fine by me. We might need to bring this back later, perhaps in a new HTTP-based API that's exposed to computes or something. But let's cross that bridge when we get there. In the meanwhile, it's good to keep the API surface as small as possible.