Basic nix fmt configuration

[pawelchcki]

Motivation

Introduces treefmt-nix and related machinery to enable custom, fast and multi language formatting.

In addition this PR leverages automated creation of Nix actions in GHa to run fast formatting scans.

Changes

Improves the nix config and adds treefmt-nix configuration Separate PRs exist which run the improved formatter on existing codebase:

Using latest python-black: https://github.com/DataDog/system-tests/pull/2599

Using shfmt and markdown formatter: https://github.com/DataDog/system-tests/pull/2602