proxy: pg17 fixes

[conradludgate]

Problem

#7809 - we do not support sslnegotiation=direct
#7810 - we do not support negotiating down the protocol extensions.

Summary of changes

1. Same as postgres, check the first startup packet byte for tls header 0x16, and check the ALPN.
2. Tell clients using protocol >3.0 to downgrade

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]

3042 tests run: 2927 passed, 0 failed, 115 skipped (full report)

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Flaky tests (1)

Postgres 15

• test_tenant_creation_fails: debug

Code coverage* (full report)

• functions: 32.6% (6934 of 21287 functions)
• lines: 50.0% (54546 of 109134 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
3830be4 at 2024-07-09T17:42:24.567Z :recycle:}

[stradig]

LGTM!