storcon: include preferred AZ in compute notifications

[jcsp]

Problem

It is unreliable for the control plane to infer the AZ for computes from where the tenant is currently attached, because if a tenant happens to be in a degraded state or a release is ongoing while a compute starts, then the tenant's attached AZ can be a different one to where it will run long-term, and the control plane doesn't check back later to restart the compute.

This can land in parallel with #9947

Summary of changes

• Thread through the preferred AZ into the compute hook code via the reconciler
• Include the preferred AZ in the body of compute hook notifications

[github-actions]

7136 tests run: 6839 passed, 0 failed, 297 skipped (full report)

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Flaky tests (3)

Postgres 17

• test_physical_replication_config_mismatch_too_many_known_xids: release-arm64
• test_timeline_archival_chaos: release-arm64, release-arm64

Code coverage* (full report)

• functions: 31.4% (8404 of 26785 functions)
• lines: 48.1% (66657 of 138684 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
670cdef at 2024-12-16T17:20:02.760Z :recycle:}