coturn: Support secrets file for configuring the static-auth-secret

[MatthewCroughan]

Motivation for this change

This PR simply rebases #75263 on master.

This change allows for the coturn shared secret to be stored in a file outside the nix store.
It also makes it easier to publish configurations in public VCS with fewer issues.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• 21.11 Release Notes (or backporting 21.05 Relase notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

[MatthewCroughan]

I have tested this as working on 21.05 on a Hetzner server I own.

[Mic92]

@GrahamcOfBorg test coturn

[MatthewCroughan]

I've adjusted the authorship and split the commits out appropriately.