Add data dir config for meilisearch

[MatthiasvB]

Extend the config of the meilisearch module to allow setting a custom location for meilisearch to store its data. This is non-trivial due to the dynamic user that's used here. Fixes #368611

Things done

• Built on platform(s)
  • x86_64-linux
  • 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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[MatthiasvB]

I have (so far) not added any tests. Will that be necessary for this change?

Also, the creation of the datadir according to the tmpfiles thingy didn't work for me doing a nixos-rebuild test. I don't know, does that need a reboot or is it somehow incorrectly done? For now, I've just created the directory manually and it does work

[MatthiasvB]

This PR fails according to formatting. I'm using Alejandra. Apparently, that's not good enough. How should I format the code?

[happysalada]

There is a nixpkgs specific formatter, its a package. It should be in the contrib docs. I would link to it but im on mobile.
Also, i think having datadir default to null is adding a bit of logic for not much gain.
Changing yhe default to be var lib meillisearch and then having the tmprule to be unconditional simplifies quite a bit.

[MatthiasvB]

Changing yhe default to be var lib meillisearch and then having the tmprule to be unconditional simplifies quite a bit.

True, it would be simpler. However, it would mean that we use "something custom built" in all cases and no longer leverage the systemd's StateDirectory in the default case. I'm not sure if that may have any downsides

[happysalada]

We could have statedirectory be an mkif with the condition that datadir = /var/lib/meilisearch for example

Statedirectory handle the creation of a directory with special permissions , thats it as far as i know. But yes, better to keep it.

[bcyran]

I think it should be dataDir by convention.