Add support for setting raw attrs

[srid]

In order to set a derivation attribute (like GIT_BIN) on a haskell package (like foo), we have no choice but to use the custom option:

          foo = {
            custom = drv: drv.overrideAttrs (oldAttrs: { } // {
              GIT_BIN = lib.getExe' pkgs.git "git";
            });
          };

But, there are two problems with this:

• custom is an overkill for this
• custom doesn't compose: Why is `settings.x.custom` expected to be unique? #430

Proposal

Add a raw option, that allows setting arbitrary attributes from the given attrset. So the above would look like:

          foo = {
            raw = {
              GIT_BIN = lib.getExe' pkgs.git "git";
            };
          };

Implementation

• This option can be added to https://github.com/srid/haskell-flake/blob/master/nix/modules/project/settings/all.nix
• Modify a suitable test under https://github.com/srid/haskell-flake/tree/master/test to test this option