storcon_cli: add 'drain' command

[VladLazar]

Problem

We need the ability to prepare a subset of storage controller managed pageservers for decommisioning. The storage controller cannot currently express this in terms of scheduling constraints (it's a pretty special case, so I'm not sure it even should).

Summary of Changes

A new drain command is added to storcon_cli. It takes a set of nodes to drain and migrates primary attachments outside of said set. Simple round robing assignment is used under the assumption that nodes outside of the draining set are evenly balanced.

Note that secondary locations are not migrated. This is fine for staging, but the migration API will have to be extended for prod in order to allow migration of secondaries as well.

I've tested this out against a neon local cluster. The immediate use for this command will be to migrate staging to ARM(Arch64) pageservers.

Related https://github.com/neondatabase/cloud/issues/14029

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]

3198 tests run: 3056 passed, 0 failed, 142 skipped (full report)

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Code coverage* (full report)

• functions: 31.6% (6626 of 20990 functions)
• lines: 48.6% (51488 of 106045 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
9ca62a5 at 2024-06-11T16:08:42.164Z :recycle:}