add: agenix flake

[ryantm]

A widely used tool for secrets deployment under NixOS.

• The namespace has a limited size agenix is a fairly long and unique name
• Common names should be avoided to prevent confusion if a user mistypes an input in their flake.nix and it resolves to another input via the registry. it is not a common name and or close to any other name in the community
• The shortcut must offer clear utility to the NixOS ecosystem; it is not intended to showcase random projects. github:ryantm/agenix is so long :wink: , definitely clear utility to be able to write agenix by itself.
• Project popularity is taken into account. has over 800 GitHub stars and its own cat memes at nixcon

[edolstra]

Can you resolve the merge conflict? Thanks!