advertise transport encoding in http transfers to

tl;dr: With this 1 line change I was able to get a speedup of 1.5x on 1Gbit/s
wan connections by enabling zstd compression in nginx.

Also nix already supported all common compression format for http
transfer, webservers usually only enable them if they are advertised
through the Accept-Encoding header.

This pull requests makes nix advertises content compression support for
zstd, br, gzip and deflate.

It's particular useful to add transparent compression for binary caches
that serve packages from the host nix store in particular nix-serve,
nix-serve-ng and harmonia.

I tried so far gzip, brotli and zstd, whereas only zstd was able to bring
me performance improvements for 1Gbit/s WAN connections.

The following nginx configuration was used in combination with the
[zstd module](https://github.com/tokers/zstd-nginx-module) and
[harmonia](https://github.com/nix-community/harmonia/)

```nix
{
  services.nginx.virtualHosts."cache.yourhost.com" = {
    locations."/".extraConfig = ''
      proxy_pass http://127.0.0.1:5000;
      proxy_set_header Host $host;
      proxy_redirect http:// https://;
      proxy_http_version 1.1;
      proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
      proxy_set_header Upgrade $http_upgrade;
      proxy_set_header Connection $connection_upgrade;

      zstd on;
      zstd_types application/x-nix-archive;
    '';
  };
}
```

For testing I unpacked a linux kernel tarball to the nix store using
this command `nix-prefetch-url --unpack https://cdn.kernel.org/pub/linux/kernel/v6.x/linux-6.1.8.tar.gz`.

Before:

```console
$ nix build && rm -rf /tmp/hello  && time ./result/bin/nix copy --no-check-sigs --from https://cache.thalheim.io --to 'file:///tmp/hello?compression=none' '/nix/store/j42mahch5f0jvfmayhzwbb88sw36fvah-linux-6.1.8.tar.gz'
warning: Git tree '/scratch/joerg/nix' is dirty

real    0m18,375s
user    0m2,889s
sys     0m1,558s
```

After:

```console
$ nix build && rm -rf /tmp/hello  && time ./result/bin/nix copy --no-check-sigs --from https://cache.thalheim.io --to 'file:///tmp/hello?compression=none' '/nix/store/j42mahch5f0jvfmayhzwb
b88sw36fvah-linux-6.1.8.tar.gz'

real    0m11,884s
user    0m4,130s
sys     0m1,439s
```

Signed-off-by: Jörg Thalheim <joerg@thalheim.io>

Update src/libstore/filetransfer.cc

Co-authored-by: Théophane Hufschmitt <7226587+thufschmitt@users.noreply.github.com>

src/libstore/filetransfer.cc

@@ -101,6 +101,7 @@ struct curlFileTransfer : public FileTransfer
                     this->result.data.append(data);
               })
         {
+            requestHeaders = curl_slist_append(requestHeaders, "Accept-Encoding: zstd, br, gzip, deflate, bzip2, xz");
             if (!request.expectedETag.empty())
                 requestHeaders = curl_slist_append(requestHeaders, ("If-None-Match: " + request.expectedETag).c_str());
             if (!request.mimeType.empty())