control_plane/attachment_service: implement PlacementPolicy::Secondary, configuration updates

[jcsp]

Problem

During onboarding, the control plane may attempt ad-hoc creation of a secondary location to facilitate live migration. This gives us two problems to solve:

• Accept 'Secondary' mode in /location_config and use it to put the tenant into secondary mode on some physical pageserver, then pass through /tenant/xyz/secondary/download requests
• Create tenants with no generation initially, since the initial Secondary mode call will not provide us a generation.

This PR also fixes modification of a tenant's TenantConf during /location_conf, which was previously ignored, and refines the flow for config modification:

• avoid bumping generations when the only reason we're reconciling an attached location is a config change
• increment TenantState.sequence when spawning a reconciler: usually schedule() does this, but when we do config changes that doesn't happen, so without this change waiters would think reconciliation was done immediately. sequence is a bit of a murky thing right now, as it's dual-purposed for tracking waiters, and for checking if an existing reconciliation is already making updates to our current sequence. I'll follow up at some point to clarify it's purpose.
• test config modification at the end of onboarding test

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]

2484 tests run: 2361 passed, 0 failed, 123 skipped (full report)

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Flaky tests (1)

Postgres 15

• test_remote_storage_upload_queue_retries: debug

Code coverage* (full report)

• functions: 28.7% (6931 of 24161 functions)
• lines: 47.2% (42560 of 90170 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
d6f6cd9 at 2024-03-01T20:23:44.355Z :recycle:}

[VladLazar]

I'm not a big fan of doing onboarding through the location config api, but as discussed out of band it makes things easier on the control plane