Support offloaded timelines during shard split

[arpad-m]

Before, we didn't copy over the index-part.json of offloaded timelines to the new shard's location, resulting in the new shard not knowing the timeline even exists.

In #9444, we copy over the manifest, but we also need to do this for index-part.json.

As the operations to do are mostly the same between offloaded and non-offloaded timelines, we can iterate over all of them in the same loop, after the introduction of a TimelineOrOffloadedArcRef type to generalize over the two cases. This is analogous to the deletion code added in #8907.

The added test also ensures that the sharded archival config endpoint works, something that has not yet been ensured by tests.

Part of #8088

[github-actions]

5273 tests run: 5059 passed, 0 failed, 214 skipped (full report)

---

Flaky tests (4)

Postgres 17

• test_location_conf_churn[1]: debug-x86-64
• test_pg_regress[None]: release-arm64
• test_readonly_node_gc: debug-x86-64
• test_ancestor_detach_branched_from[True-True-at]: debug-x86-64

Code coverage* (full report)

• functions: 31.4% (7664 of 24427 functions)
• lines: 48.8% (60268 of 123425 lines)

* collected from Rust tests only

---

_{The comment gets automatically updated with the latest test results
7b558cc at 2024-10-24T15:06:38.557Z :recycle:}

[jcsp]

This looks correct, but the way OffloadedTimeline::remote_client is used is a bit tenuous & I think other stuff that uses it might not work (comment inline)