storcon: use the SchedulingPolicy enum in SafekeeperPersistence

[arpad-m]

We don't want to serialize to/from string all the time, so use SchedulingPolicy in SafekeeperPersistence via the use of a wrapper.

Stacked atop #10891

[github-actions]

7557 tests run: 7184 passed, 0 failed, 373 skipped (full report)

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Code coverage* (full report)

• functions: 32.9% (8621 of 26205 functions)
• lines: 48.8% (72725 of 148913 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
a1f4091 at 2025-02-20T16:53:56.751Z :recycle:}

[arssher]

Can we implement FromSql directly on SkSchedulingPolicy to avoid messing with wrapper type?

Also please rebase/merge so for easier review, it seems to have code from prev PR.

[arpad-m]

Both FromSql and SkSchedulingPolicy come from third party crates, so we can't implement it in our crate. I don't think that adding a "heavy" dependency like diesel to safekeeper_api is a good idea, so that's why I added a wrapper type.

[arpad-m]

I've added a doc comment to explain the reason why we have a wrapper.

[arssher]

Hang on, why SkSchedulingPolicy is even in pageserver_api crate? Let's just move it straightly to storcon. Safekeepers themselves don't need to be aware how they are scheduled, even more pageservers.