Add a script to add missing license headers (#1094)

Work towards #916

addlicense.sh

@@ -0,0 +1,23 @@
+#!/bin/bash
+# Copyright 2023 The Bazel Authors. All rights reserved.
+#
+# 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.
+
+
+if ! command -v addlicense @>&1 >/dev/null; then
+    echo "ERROR: addlicense not installed."
+    echo "Install using https://github.com/google/addlicense#install"
+    exit 1
+fi
+
+addlicense -v -l apache -c 'The Bazel Authors. All rights reserved.' "$@"