proxy: async aware password validation

[conradludgate]

Problem

spawn_blocking in #7171 was a hack

Summary of changes

neondatabase/rust-postgres#29

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]

2706 tests run: 2574 passed, 0 failed, 132 skipped (full report)

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Flaky tests (1)

Postgres 16

• test_compute_pageserver_connection_stress: release

Code coverage* (full report)

• functions: 28.3% (7132 of 25198 functions)
• lines: 46.8% (43702 of 93283 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
98e5f0b at 2024-03-20T13:13:23.770Z :recycle:}