Add safekeeper utilization endpoint

[arpad-m]

Add an endpoint to obtain the utilization of a safekeeper. Future changes to the storage controller can use this endpoint to find the most suitable safekeepers for newly created timelines, analogously to how it's done for pageservers already.

Initially we just want to assign by timeline count, then we can iterate from there.

Part of #9011

[github-actions]

7326 tests run: 6949 passed, 0 failed, 377 skipped (full report)

---

Flaky tests (3)

Postgres 15

• test_compute_pageserver_connection_stress: release-arm64
• test_isolation[None]: release-arm64
• test_physical_replication_config_mismatch_max_locks_per_transaction: release-arm64

Code coverage* (full report)

• functions: 33.7% (8429 of 25046 functions)
• lines: 49.2% (70492 of 143362 lines)

* collected from Rust tests only

---

_{The comment gets automatically updated with the latest test results
26e3f61 at 2025-01-16T17:59:48.845Z :recycle:}