feat: Add nix flake setup with envrc

[Anderssorby]

Description

This adds a nix flake which makes building and deploying much easier. It provides a rust toolchain and loads all necessary dependencies into the environment which can be loaded automatically with direnv.

Type of change

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

How Has This Been Tested?

The node builds without any additional installation with cargo build inside the nix develop environment and with nix build for fully content addressed builds.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules