✨ Add support for ASGI root_path for openapi docs (#1199)

* Use ASGI root_path when it is provided and openapi_prefix is empty.

* Strip trailing slashes from root_path.

* Please mypy.

* Fix extending openapi test.

* 📝 Add docs and tutorial for using root_path behind a proxy

* ♻️ Refactor application root_path logic, use root_path, deprecate openapi_prefix

* ✅ Add tests for Behind a Proxy with root_path

* ♻️ Refactor test

* 📝 Update/add docs for Sub-applications and Behind a Proxy

* 📝 Update Extending OpenAPI with openapi_prefix parameter

* ✅ Add test for deprecated openapi_prefix

Co-authored-by: Sebastián Ramírez <tiangolo@gmail.com>

docs_src/behind_a_proxy/tutorial002.py

@@ -0,0 +1,8 @@
+from fastapi import FastAPI, Request
+
+app = FastAPI(root_path="/api/v1")
+
+
+@app.get("/app")
+def read_main(request: Request):
+    return {"message": "Hello World", "root_path": request.scope.get("root_path")}