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/values.go

@@ -0,0 +1,19 @@
+package runtime
+
+// Values typically represent parameters on a http request.
+type Values map[string][]string
+
+// GetOK returns the values collection for the given key.
+// When the key is present in the map it will return true for hasKey.
+// When the value is not empty it will return true for hasValue.
+func (v Values) GetOK(key string) (value []string, hasKey bool, hasValue bool) {
+	value, hasKey = v[key]
+	if !hasKey {
+		return
+	}
+	if len(value) == 0 {
+		return
+	}
+	hasValue = true
+	return
+}