proxy: ensure startup params are properly forwarded to compute

[conradludgate]

Problem

#1287 proxy does not forward all params to compute

Summary of changes

• refactor startup messages to have less small allocs

actual solution: todo, will open a PR in rust-postgres fork

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]

2382 tests run: 2268 passed, 0 failed, 114 skipped (full report)

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Flaky tests (3)

Postgres 15

• test_crafted_wal_end[last_wal_record_crossing_segment]: release
• test_sharding_service_smoke: debug

Postgres 14

• test_gc_cutoff: debug

Code coverage (full report)

• functions: 54.6% (11286 of 20678 functions)
• lines: 81.6% (63640 of 77951 lines)

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_{The comment gets automatically updated with the latest test results
db7b244 at 2024-02-02T19:28:26.137Z :recycle:}