Configurable username regex

[tanneberger]

Changes

This pull request makes the Username Regex configurable over an environment variable.

Motivation

Our organization wants to enforce some username schema with firstname.secondname and currently . are not allowed by the posix user regex. With those changes, we can configure portunus to our username schema

• PORTUNUS_POSIX_ACCOUNT_REGEX which takes the new username regex

Tested

• Compiling
• Adjusted Readme
• Rolled out on your nixos maschine:
  • added regexOption here https://github.com/NixOS/nixpkgs/commit/535b4ba699ad76ef3d57a33ac8415a4bec736dfe which I try to yeet into nixpkgs after this pr is done

[majewsky]

Implemented differently and more thoroughly on top of the new validation code in 210726b7a7816ca28ab8180f0307e0939b38f0cc.