Added sample for using custom server. (GoogleCloudPlatform#1833)

Author: chenyumic

appengine/standard_python37/custom-server/app.yaml

@@ -0,0 +1,4 @@
+# [START gae_python37_custom_runtime]
+runtime: python37
+entrypoint: uwsgi --http-socket :8080 --wsgi-file main.py --callable app --master --processes 1 --threads 2
+# [END gae_python37_custom_runtime]

appengine/standard_python37/custom-server/main.py

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+# Copyright 2018 Google LLC
+#
+# 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.
+
+from flask import Flask
+
+app = Flask(__name__)
+
+
+@app.route('/')
+def main():
+    return 'Hello, World!'
+
+
+if __name__ == '__main__':
+    app.run(debug=True)

appengine/standard_python37/custom-server/requirements.txt

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+uwsgi==2.0.17.1
+flask==1.0.2