fix: add safekeeper team to pgxn codeowners

[skyzh]

Problem

pgxn/ also contains WAL proposer code, so modifications to this directory should be able to be approved by the safekeeper team.

Summary of changes

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

[skyzh]

e.g., https://github.com/neondatabase/neon/commits/main/pgxn/neon/walproposer.c is part of the safekeeper

[jcsp]

@arssher wdyt? does it make sense to require compute team review for changes to the C code that we link into postgres, or would you like your team to be able to do these changes unilaterally?

The other side effect of this change would be that your team would get auto-assigned as reviewers for pgxn code that is unrelated to the safekeeper client code, and be responsible for either reviewing or delegating that.

[github-actions]

2706 tests run: 2576 passed, 0 failed, 130 skipped (full report)

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Flaky tests (1)

Postgres 14

• test_delete_timeline_client_hangup: release

Code coverage* (full report)

• functions: 28.3% (7132 of 25198 functions)
• lines: 46.9% (43713 of 93290 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
28acbda at 2024-03-20T15:35:28.728Z :recycle:}

[arssher]

wdyt? does it make sense to require compute team review for changes to the C code that we link into postgres, or would you like your team to be able to do these changes unilaterally?

We definitely do want to be able to make them unilaterally; if this is not the case, I just manually ask for the review.

The other side effect of this change would be that your team would get auto-assigned as reviewers for pgxn code that is unrelated

True, but there is no easy ideal solution. Even without it generally in my experience this codeowners thing quite frequently assigns unsuitable reviewers, but previous point (being able to merge without unrelated team reviews) is more important.

[arssher]

We should also add @neondatabase/safekeepers to /libs/postgres_ffi/. In fact likely this PR is caused by
#7143