Add neon.lakebase_mode boolean GUC

[tristan957]

This GUC will become useful for temporarily disabling Lakebase-specific features during the code merge.

[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.

[github-actions]

8991 tests run: 8320 passed, 0 failed, 671 skipped (full report)

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Flaky tests (6)

Postgres 17

• test_import_timeline_disk_pressure_eviction: release-arm64-without-lfc
• test_hot_standby_gc[True]: release-x86-64-with-lfc, release-x86-64-without-lfc
• test_ps_unavailable_after_delete[DeletionAPIKind.FORCE]: release-arm64-without-lfc

Postgres 15

• test_hot_standby_gc[True]: release-x86-64-with-lfc

Postgres 14

• test_ps_unavailable_after_delete[DeletionAPIKind.FORCE]: release-arm64-with-lfc

Code coverage* (full report)

• functions: 34.7% (8805 of 25340 functions)
• lines: 45.8% (71298 of 155612 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
fadde0b at 2025-07-23T22:33:41.064Z :recycle:}