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/loads/options.go

@@ -0,0 +1,61 @@
+package loads
+
+type options struct {
+	loader *loader
+}
+
+func defaultOptions() *options {
+	return &options{
+		loader: loaders,
+	}
+}
+
+func loaderFromOptions(options []LoaderOption) *loader {
+	opts := defaultOptions()
+	for _, apply := range options {
+		apply(opts)
+	}
+
+	return opts.loader
+}
+
+// LoaderOption allows to fine-tune the spec loader behavior
+type LoaderOption func(*options)
+
+// WithDocLoader sets a custom loader for loading specs
+func WithDocLoader(l DocLoader) LoaderOption {
+	return func(opt *options) {
+		if l == nil {
+			return
+		}
+		opt.loader = &loader{
+			DocLoaderWithMatch: DocLoaderWithMatch{
+				Fn: l,
+			},
+		}
+	}
+}
+
+// WithDocLoaderMatches sets a chain of custom loaders for loading specs
+// for different extension matches.
+//
+// Loaders are executed in the order of provided DocLoaderWithMatch'es.
+func WithDocLoaderMatches(l ...DocLoaderWithMatch) LoaderOption {
+	return func(opt *options) {
+		var final, prev *loader
+		for _, ldr := range l {
+			if ldr.Fn == nil {
+				continue
+			}
+
+			if prev == nil {
+				final = &loader{DocLoaderWithMatch: ldr}
+				prev = final
+				continue
+			}
+
+			prev = prev.WithNext(&loader{DocLoaderWithMatch: ldr})
+		}
+		opt.loader = final
+	}
+}