Dynamic Masking Support for anon v2

[thesuhas]

Problem

This PR works on adding dynamic masking support for anon v2. It currently only supports static masking.

Summary of changes

Added a security definer function that sets the dynamic masking guc to true with superuser permissions.
Added a security definer function that adds anon to session_preload_libraries if it's not already present.

Related to: https://github.com/neondatabase/cloud/issues/20456

[github-actions]

8349 tests run: 7844 passed, 0 failed, 505 skipped (full report)

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Flaky tests (3)

Postgres 16

• test_pg_regress[v2-4]: release-arm64-with-lfc

Postgres 15

• test_compute_pageserver_connection_stress: release-x86-64-with-lfc

Postgres 14

• test_partial_evict_tenant[relative_equal]: release-x86-64-with-lfc

Code coverage* (full report)

• functions: 32.7% (8970 of 27405 functions)
• lines: 48.9% (78277 of 160115 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
2620485 at 2025-05-01T22:53:11.504Z :recycle:}

[MMeent]

As mentioned before, there are IMO too many places where anon is added by default. I'd rather not add these moving parts by default; AFAIK it is going to be an opt-in feature, and not forced on all users.