By default in Nginx, if you want to override a single fastcgi_param,
you have to override all of them. This is less of a big deal if
you're editing the Nginx configuration directly, but when you're
generating the Nginx configuration with Nix it can be very annoying to
bloat your configuration repeating the default values of FastCGI
parameters every time.

This patch adds a fastcgiParams option to Nginx locations. If any
parameters are set through this, all the default values will be
included as well, so only the ones that are changing need to be
supplied. There's no way to use fastcgiParams to actually override
all parameters if that's what you want, but I think that's a niche use
case and it's still possible using extraConfig, which up until now was
the only option

Nginx allows the fastcgi_param directive in http and server scopes as
well as location, but here I only support location. It would be
possible to support the others, but I don't think it's worth it. It
would be a possible future enhancement if somebody has a need for it.

Motivation for this change

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/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.