✨ 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/extending_openapi/tutorial001.py

@@ -9,7 +9,7 @@ async def read_items():
     return [{"name": "Foo"}]
 
 
-def custom_openapi():
+def custom_openapi(openapi_prefix: str):
     if app.openapi_schema:
         return app.openapi_schema
     openapi_schema = get_openapi(
@@ -17,6 +17,7 @@ def custom_openapi():
         version="2.5.0",
         description="This is a very custom OpenAPI schema",
         routes=app.routes,
+        openapi_prefix=openapi_prefix,
     )
     openapi_schema["info"]["x-logo"] = {
         "url": "https://fastapi.tiangolo.com/img/logo-margin/logo-teal.png"