Add support for Fulcio and Rekor, and --sign-by-sigstore=param-file

(skopeo copy) and (skopeo sync) now support --sign-by-sigstore=param-file,
using the containers-sigstore-signing-params.yaml(5) file format.

That notably adds support for Fulcio and Rekor signing.

Signed-off-by: Miloslav Trmač <mitr@redhat.com>

vendor/github.com/skratchdot/open-golang/open/exec_darwin.go

@@ -0,0 +1,15 @@
+// +build darwin
+
+package open
+
+import (
+	"os/exec"
+)
+
+func open(input string) *exec.Cmd {
+	return exec.Command("open", input)
+}
+
+func openWith(input string, appName string) *exec.Cmd {
+	return exec.Command("open", "-a", appName, input)
+}