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/go-openapi/runtime/middleware/denco/util.go

@@ -0,0 +1,12 @@
+package denco
+
+// NextSeparator returns an index of next separator in path.
+func NextSeparator(path string, start int) int {
+	for start < len(path) {
+		if c := path[start]; c == '/' || c == TerminationCharacter {
+			break
+		}
+		start++
+	}
+	return start
+}