[feature][python-flask] Add CORS support to python-flask server (#8472)

* Add CORS support to python-flask server generator

* Documentation update and CORS support for other generators using the same base class

* Trivial sample changes

samples/server/petstore/python-flask/openapi_server/__main__.py

@@ -11,6 +11,7 @@ def main():
     app.add_api('openapi.yaml',
                 arguments={'title': 'OpenAPI Petstore'},
                 pythonic_params=True)
+
     app.run(port=8080)