Secret Management for the spotify connect token

[Mcrtin]

Is your feature already implemented in the latest master?
I don't think so, but please show me otherwise.

Is your feature request related to a problem? Please describe.
I'm using Spotify-player with home-manager for a declarative config. However, I don't want to leak my secrets in my config, because the get evaluated and put into the world-readable nix store.

Describe the solution you'd like
Add a 'token command' field like spotifyd's 'password_cmd' or neomutt's 'account_command' (https://neomutt.org/feature/account-cmd)

Describe alternatives you've considered
Not putting the key in the upstream repo.

Additional context
I'm using nixos + home-manager + sops-nix

[aome510]

For secret, do you mean client_id? Should be addressed by #548