feat: add an option to install shorebird with nix

[mikeborodin]

Description

Install shorebird with Nix. I think this would broaden the user base of shorebird if we include Nix users and would allow to integrate it with other tools that use Nix under the hood like devbox or for example idx.dev Requirements

• [ ] add flake.nix configuration file so that the users can use it to their dev environment with devbox add github:shorebirdtech/shorebird

Additional Context I would be ready to submit a pr if this feature is welcome, thank you:)