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/validate/context.go

@@ -0,0 +1,56 @@
+package validate
+
+import (
+	"context"
+)
+
+// validateCtxKey is the key type of context key in this pkg
+type validateCtxKey string
+
+const (
+	operationTypeKey validateCtxKey = "operationTypeKey"
+)
+
+type operationType string
+
+const (
+	request  operationType = "request"
+	response operationType = "response"
+	none     operationType = "none" // not specified in ctx
+)
+
+var operationTypeEnum = []operationType{request, response, none}
+
+// WithOperationRequest returns a new context with operationType request
+// in context value
+func WithOperationRequest(ctx context.Context) context.Context {
+	return withOperation(ctx, request)
+}
+
+// WithOperationRequest returns a new context with operationType response
+// in context value
+func WithOperationResponse(ctx context.Context) context.Context {
+	return withOperation(ctx, response)
+}
+
+func withOperation(ctx context.Context, operation operationType) context.Context {
+	return context.WithValue(ctx, operationTypeKey, operation)
+}
+
+// extractOperationType extracts the operation type from ctx
+// if not specified or of unknown value, return none operation type
+func extractOperationType(ctx context.Context) operationType {
+	v := ctx.Value(operationTypeKey)
+	if v == nil {
+		return none
+	}
+	res, ok := v.(operationType)
+	if !ok {
+		return none
+	}
+	// validate the value is in operation enum
+	if err := Enum("", "", res, operationTypeEnum); err != nil {
+		return none
+	}
+	return res
+}