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/pre_go18.go

@@ -0,0 +1,9 @@
+// +build !go1.8
+
+package middleware
+
+import "net/url"
+
+func pathUnescape(path string) (string, error) {
+	return url.QueryUnescape(path)
+}