feat(sgbackstage): add CLI tool

Can validate Backstage catalog manifests using the Backstage JSON
schemas.

tools/sgbackstage/tools.go

@@ -0,0 +1,51 @@
+package sgbackstage
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"runtime"
+
+	"go.einride.tech/sage/sg"
+	"go.einride.tech/sage/sgtool"
+)
+
+const (
+	version    = "0.6.0"
+	binaryName = "backstage"
+)
+
+func Command(ctx context.Context, args ...string) *exec.Cmd {
+	sg.Deps(ctx, PrepareCommand)
+	return sg.Command(ctx, sg.FromBinDir(binaryName), args...)
+}
+
+func PrepareCommand(ctx context.Context) error {
+	binDir := sg.FromToolsDir(binaryName, version)
+	binary := filepath.Join(binDir, binaryName)
+	hostOS := runtime.GOOS
+	hostArch := runtime.GOARCH
+	downloadURL := fmt.Sprintf(
+		"https://github.com/einride/backstage-go/releases/download/v%s/backstage-go_%s_%s_%s.tar.gz",
+		version,
+		version,
+		hostOS,
+		hostArch,
+	)
+	if err := sgtool.FromRemote(
+		ctx,
+		downloadURL,
+		sgtool.WithDestinationDir(binDir),
+		sgtool.WithUntarGz(),
+		sgtool.WithSkipIfFileExists(binary),
+		sgtool.WithSymlink(binary),
+	); err != nil {
+		return fmt.Errorf("unable to download %s: %w", binaryName, err)
+	}
+	if err := os.Chmod(binary, 0o755); err != nil {
+		return fmt.Errorf("unable to make %s command: %w", binaryName, err)
+	}
+	return nil
+}