fix: warnings when compiling neon extensions

[skyzh]

Problem

proceeding #7010, close #6188

Summary of changes

This pull request (should) fix all warnings except -Wdeclaration-after-statement in the neon extension compilation.

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]

2496 tests run: 2373 passed, 0 failed, 123 skipped (full report)

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Flaky tests (1)

Postgres 14

• test_crafted_wal_end[wal_record_crossing_segment_followed_by_small_one]: debug

Code coverage* (full report)

• functions: 28.8% (7032 of 24443 functions)
• lines: 47.6% (43449 of 91295 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
b1de758 at 2024-03-11T18:03:21.498Z :recycle:}