libpq: init at 17.2

[wolfgangwalther]

This creates a separate libpq package as discussed in #61580 (comment).

This brings down the number of rebuilds after updating postgresql to about 1.2k darwin and 2.5k linux. Before, they were ~ around 5k, IIRC.

Unfortunately, we are still not anywhere low enough to be able to merge postgresql updates directly into master. I think the nr. 1 reason for that is postgresqlTestHook - and the fact that many packages use a very simple postgresql server in their build dependencies for the check phase.

The only way to get the number of rebuilds lower would be to package postgresqlTestHook as a separate postgresql derivation. The idea would be, that security related issues are irrelevant for the check phase of other packages - and thus updating this separately could easily go through staging with a certain delay. This derivation could also be one that is built with the minimal feature set, making it slimmer overall.

I still think that introducing libpq is a good thing on it's own, thus this PR.

Closes #61580 and #191920

Things done

• Built on platform(s)
  • x86_64-linux (the directly changed packages)
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 25.05 Release Notes (or backporting 24.11 and 25.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[wolfgangwalther]

Also builds on pkgsStatic now and thus solves #191920, too.

[szlend]

Cross-compiles from aarch64-darwin -> aarch64-linux/x86_64-linux perfectly, compared to postgresql.lib which is failing again.

I think this is a huge improvement for reasons mentioned in #61580 (comment). Both for sustainability by reducing the number of unnecessary rebuilds and reducing complexity when cross-compiling or statically linking.