feat: add a nix default package and developer shell for `gotrue`

[yvan-sraka]

This PR makes some fixes and tweaks on top of https://github.com/supabase/auth/pull/2020 (it's used for working on the system-manager modules integration in https://github.com/supabase/postgres/pull/1787).

The flake have been rewritten using blueprint framework, it now provides a straightforward devshell, a default package (the binary built from the Go sources in this repository), and an auth NixOS module with default settings that are turned into the required configuration files and a systemd service. Some utility scripts for managing the service have been removed, as we now expect this to be handled directly through systemd utilities.