feat(tvix/eval): Implement builtins.fetchTarball

Implement a first pass at the fetchTarball builtin.

This uses much of the same machinery as fetchUrl, but has the extra
complexity that tarballs have to be extracted and imported as store
paths (into the directory- and blob-services) before hashing. That's
reasonably involved due to the structure of those two services.

This is (unfortunately) not easy to test in an automated way, but I've
tested it manually for now and it seems to work:

    tvix-repl> (import ../. {}).third_party.nixpkgs.hello.outPath
    => "/nix/store/dbghhbq1x39yxgkv3vkgfwbxrmw9nfzi-hello-2.12.1" :: string

Co-authored-by: Connor Brewster <cbrewster@hey.com>
Change-Id: I57afc6b91bad617a608a35bb357861e782a864c8
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11020
Autosubmit: aspen <root@gws.fyi>
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

users/picnoir/tvix-daemon/Cargo.nix

@@ -2366,8 +2366,7 @@ rec {
           }
         ];
         features = {
-          "async" = [ "futures-util" ];
-          "futures-util" = [ "dep:futures-util" ];
+          "async" = [ "tokio" ];
           "pin-project-lite" = [ "dep:pin-project-lite" ];
           "tokio" = [ "dep:tokio" ];
           "wire" = [ "tokio" "pin-project-lite" ];