lib/attrsets: add filterAttrsRecursiveCond function

[r-burns]

based on the mapAttrsRecursive{,Cond} functions

Motivation for this change

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• 21.11 Release Notes (or backporting 21.05 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.

[roberth]

Please add tests to lib/tests/misc.nix

[wolfgangwalther]

I took the liberty to rebase this PR on latest master. No other changes.

[roberth]

Please forgive my short review which is just an extension of my earlier comment. I hope it helps. Otherwise, as it's been a while, if you do not wish to work on this, you can say so and let anyone else hopefully complete it.

This PR certainly needs tests, and the documentation could be improved, just as the standard for docs is improving.
For both, I'd like to refer to #395160

[r-burns]

Thanks, yeah it's been so long since I opened this that to be honest I don't remember what I was going to use it for 😅

I think the principle of this function is correct, so I'll just leave it up in case someone else finds it useful and wants to take over. Feel free to mark as draft, close, push, etc.