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/golang.org/x/oauth2/internal/transport.go

@@ -0,0 +1,33 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package internal
+
+import (
+	"context"
+	"net/http"
+)
+
+// HTTPClient is the context key to use with golang.org/x/net/context's
+// WithValue function to associate an *http.Client value with a context.
+var HTTPClient ContextKey
+
+// ContextKey is just an empty struct. It exists so HTTPClient can be
+// an immutable public variable with a unique type. It's immutable
+// because nobody else can create a ContextKey, being unexported.
+type ContextKey struct{}
+
+var appengineClientHook func(context.Context) *http.Client
+
+func ContextClient(ctx context.Context) *http.Client {
+	if ctx != nil {
+		if hc, ok := ctx.Value(HTTPClient).(*http.Client); ok {
+			return hc
+		}
+	}
+	if appengineClientHook != nil {
+		return appengineClientHook(ctx)
+	}
+	return http.DefaultClient
+}