chore(proxy/jwks): reduce the rightward drift of jwks renewal

[conradludgate]

I found the rightward drift of the renew_jwks function hard to review.

This PR splits out some major logic and uses early returns to make the happy path more linear.

[github-actions]

5561 tests run: 5344 passed, 0 failed, 217 skipped (full report)

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Flaky tests (2)

Postgres 17

• test_location_conf_churn[1]: debug-x86-64

Postgres 14

• test_wal_restore_initdb: release-arm64

Code coverage* (full report)

• functions: 31.4% (7955 of 25340 functions)
• lines: 49.3% (63102 of 128007 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
2f1cdea at 2024-11-22T15:02:06.714Z :recycle:}