Add a script to generate a bundle of plugins for CocoaPods (#872)

Motivation:

https://github.com/grpc/grpc-swift/pull/869 adds a plugin containing the
protoc binaries for CocoaPods. It relies on having a zip of the binaries
in the tagged release.

Modifications:

- Add a script to generate a zip of the protoc binaries.

Result:

We can generate a bundle of plugins which may be pulled in by a
CocoaPod.

scripts/bundle-plugins-for-release.sh

@@ -0,0 +1,66 @@
+#!/bin/bash
+
+# Copyright 2020, gRPC 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.
+
+set -eu
+
+# This script bundles up the gRPC and Protobuf protoc plugins into a zip file
+# suitable for the 'gRPC-Swift-Plugins' CocoaPod.
+#
+# The contents of thie zip should look like this:
+#
+#   ├── LICENSE
+#   └── bin
+#       ├── protoc-gen-grpc-swift
+#       └── protoc-gen-swift
+#
+# Note: the name of the generated zip should match that expected by the
+# buid_podspecs.py script.
+
+if [[ $# -lt 1 ]]; then
+  echo "Usage: $0 RELEASE_VERSION"
+  exit 1
+fi
+
+version=$1
+zipfile="protoc-grpc-swift-plugins-${version}.zip"
+
+# Where are we?
+here="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
+# The root of the repo is just above us.
+root="${here}/.."
+
+# Make a staging area.
+stage=$(mktemp -d)
+stage_bin="${stage}/bin"
+mkdir -p "${stage_bin}"
+
+# Make the plugins.
+make -C "${root}" plugins
+
+# Copy them to the stage.
+cp "${root}/protoc-gen-grpc-swift" "${stage_bin}"
+cp "${root}/protoc-gen-swift" "${stage_bin}"
+
+# Copy the LICENSE to the stage.
+cp "${root}/LICENSE" "${stage}"
+
+# Zip it up.
+pushd "${stage}" || exit
+zip -r "${zipfile}" .
+popd || exit
+
+# Tell us where it is.
+echo "Created ${stage}/${zipfile}"