Add a script to download and stage artifacts (#65)

### Motivation & Modifications

Add a script to download and stage artifacts so that we can just use a
`svn add *` command to add them to the central repo.

### Verification

Take the 3.0.0-candidate-3 release as example, after configuring the
`GITHUB_TOKEN` environment variable, you only need to run:

```bash
# See https://github.com/apache/pulsar-client-python/actions/runs/3709463737
$PROJECT_DIR/build-support/stage-release.sh v3.1.0-candidate-3 3709463737
```

Then the layout of the current directory will be the same as
https://dist.apache.org/repos/dist/dev/pulsar/pulsar-client-python-3.0.0-candidate-3/.

Author: BewareMyPower

build-support/stage-release.sh

@@ -0,0 +1,78 @@
+#!/usr/bin/env bash
+#
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you 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 -e
+
+if [[ $# -lt 2 ]]; then
+    echo "Usage: $0 \$VERSION \$WORKFLOW_ID"
+    exit 1
+fi
+if [[ ! $GITHUB_TOKEN ]]; then
+    echo "GITHUB_TOKEN (must have the actions scope) is not set"
+    exit 2
+fi
+
+VERSION=$1
+TAG=v$VERSION
+WORKFLOW_ID=$2
+
+# Download the source tar
+curl -O -L https://github.com/apache/pulsar-client-python/archive/refs/tags/$TAG.tar.gz
+mv $TAG.tar.gz pulsar-client-python-$VERSION.tar.gz
+
+# Download the Python wheels
+URLS=$(curl -L https://api.github.com/repos/apache/pulsar-client-python/actions/runs/$WORKFLOW_ID/artifacts \
+  | jq '.artifacts[] .archive_download_url' | sed 's/^"\(.*\)"$/\1/')
+for URL in $URLS; do
+    curl -O -L $URL -H "Accept: application/vnd.github+json" -H "Authorization: Bearer $GITHUB_TOKEN"
+    unzip -q zip
+    rm -f zip
+done
+
+sign() {
+    FILE=$1
+    gpg --armor --output $FILE.asc --detach-sig $FILE
+    shasum -a 512 $FILE > $FILE.sha512
+}
+
+export GPG_TTY=$(tty)
+set -x
+for WHEEL in $(ls *.whl); do
+    sign $WHEEL
+done
+sign pulsar-client-python-$VERSION.tar.gz
+
+mkdir windows && cd windows
+mv ../pulsar_client*win*.whl* . && cd ..
+
+mkdir macos && cd macos
+mv ../pulsar_client*macos*.whl* . && cd ..
+
+mkdir linux-glibc-x86_64 && cd linux-glibc-x86_64
+mv ../pulsar_client*manylinux*x86_64.whl* . && cd ..
+
+mkdir linux-glibc-arm64 && cd linux-glibc-arm64
+mv ../pulsar_client*manylinux*aarch64.whl* . && cd ..
+
+mkdir linux-musl-x86_64 && cd linux-musl-x86_64
+mv ../pulsar_client*musllinux*x86_64.whl* . && cd ..
+
+mkdir linux-musl-arm64 && cd linux-musl-arm64
+mv ../pulsar_client*musllinux*aarch64.whl* . && cd ..