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/spec/url_go19.go

@@ -0,0 +1,14 @@
+//go:build go1.19
+// +build go1.19
+
+package spec
+
+import "net/url"
+
+func parseURL(s string) (*url.URL, error) {
+	u, err := url.Parse(s)
+	if err == nil {
+		u.OmitHost = false
+	}
+	return u, err
+}