pageserver: update mgmt_api to use TenantShardId

[jcsp]

Problem

The API client was written around the same time as some of the server APIs changed from TenantId to TenantShardId

Closes: #6154

Summary of changes

• Refactor mgmt_api timeline_info and keyspace methods to use TenantShardId to match the server

This doesn't make pagebench sharding aware, but it paves the way to do so later.

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]

2748 tests run: 2624 passed, 0 failed, 124 skipped (full report)

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Code coverage* (full report)

• functions: 27.9% (6380 of 22848 functions)
• lines: 46.8% (44982 of 96060 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
3b66900 at 2024-04-04T12:10:22.268Z :recycle:}