Update examples for Connexion 3.0 (#1615)

This PR updates the examples for Connexion 3.0 and merges them for
OpenAPI and Swagger.

2 examples required some changes to make them work:
- The reverse proxy example required some fixes to the
SwaggerUIMiddleware to leverage the `root_path` correctly. This is
included in the PR.
- The enforced defaults example requires the json validator to adapt the
body and pass it on. We currently pass on the original body after
validation, and I'm not sure if we should change this. I'll submit a
separate PR to discuss this.

examples/helloworld/hello.py

@@ -0,0 +1,16 @@
+from pathlib import Path
+
+import connexion
+
+
+def post_greeting(name: str) -> str:
+    return f"Hello {name}"
+
+
+app = connexion.FlaskApp(__name__, specification_dir="spec/")
+app.add_api("openapi.yaml", arguments={"title": "Hello World Example"})
+app.add_api("swagger.yaml", arguments={"title": "Hello World Example"})
+
+
+if __name__ == "__main__":
+    app.run(f"{Path(__file__).stem}:app", port=8080)