Add a dummy.go file to allow vendoring config (sigstore#1520)

This is a placeholder that allows us to pull in config files via go mod vendor.

Signed-off-by: Josh Dolitsky <josh@dolit.ski>

config/dummy.go

@@ -0,0 +1,17 @@
+// Copyright 2022 The Sigstore Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package config is a placeholder that allows us to pull in config files
+// via go mod vendor.
+package config