Download extension if it does not exist on the filesystem

[tristan957]

Previously we attempted to download all extensions in CREATE EXTENSION statements. Extensions like pg_stat_statements and neon are not remote extensions, but still we were requesting them when skip_pg_catalog_updates was set to false.

Fixes: #11127

[github-actions]

If this PR added a GUC in the Postgres fork or neon extension,
please regenerate the Postgres settings in the cloud repo:

make NEON_WORKDIR=path/to/neon/checkout \
  -C goapp/internal/shareddomain/postgres generate

If you're an external contributor, a Neon employee will assist in
making sure this step is done.

[tristan957]

I'd love to add a test for this, but we would need to add some more metrics. compute_ctl_remote_ext_requests_total doesn't cut it because it is collected to deep in the stack. @ololobus I think when we add axum metrics, that will be a good time to add a test for this.

[github-actions]

8041 tests run: 7658 passed, 0 failed, 383 skipped (full report)

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Flaky tests (4)

Postgres 17

• test_lsn_lease_storcon: debug-x86-64-without-lfc

Postgres 16

• test_sliding_working_set_approximation: release-arm64-with-lfc

Postgres 15

• test_sliding_working_set_approximation: release-arm64-with-lfc
• test_pageserver_restarts_under_worload: release-arm64-with-lfc

Code coverage* (full report)

• functions: 32.3% (8730 of 27067 functions)
• lines: 48.5% (75319 of 155338 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
6a14b07 at 2025-03-31T23:20:59.123Z :recycle:}

[ololobus]

@ololobus I think when we add axum metrics, that will be a good time to add a test for this.

Yeah, I think we could add axum / compute_ctl http server metrics and that will cover this case