[#733] neofs-adm: fetch contracts release from Github

Signed-off-by: Evgenii Stratonikov <evgeniy@nspcc.ru>

cmd/neofs-adm/internal/modules/morph/download.go

@@ -0,0 +1,50 @@
+package morph
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"io"
+	"net/http"
+	"os"
+	"strings"
+
+	"github.com/google/go-github/v39/github"
+	"github.com/spf13/cobra"
+)
+
+func downloadContractsFromGithub(cmd *cobra.Command) (string, error) {
+	gcl := github.NewClient(nil)
+	release, _, err := gcl.Repositories.GetLatestRelease(context.Background(), "nspcc-dev", "neofs-contract")
+	if err != nil {
+		return "", fmt.Errorf("can't fetch release info: %w", err)
+	}
+
+	cmd.Printf("Found %s (%s), downloading...\n", release.GetTagName(), release.GetName())
+
+	var url string
+	for _, a := range release.Assets {
+		if strings.HasPrefix(a.GetName(), "neofs-contract") {
+			url = a.GetBrowserDownloadURL()
+			break
+		}
+	}
+	if url == "" {
+		return "", errors.New("can't find contracts archive in release assets")
+	}
+
+	resp, err := http.Get(url)
+	if err != nil {
+		return "", fmt.Errorf("can't fetch contracts archive: %w", err)
+	}
+	defer resp.Body.Close()
+
+	f, err := os.CreateTemp("", "neofs-contract-*.tar.gz")
+	if err != nil {
+		return "", fmt.Errorf("can't allocate temporary file: %w", err)
+	}
+	defer f.Close()
+
+	_, err = io.Copy(f, resp.Body)
+	return f.Name(), err
+}