Implement user facing interface for ConnexionMiddleware (#1621)

This PR adds an interface for the ConnexionMiddleware, similar to the
interface of the Connexion Apps.

The Connexion Apps are now a simple wrapper around the
ConnexionMiddleware and framework app, delegating the work to the
middleware. This enables a similar interface and behavior for users when
using either the middleware or apps.

The arguments are repeated everywhere there is a user interface, but are
parsed in a central place. Repeating the arguments is not DRY, but
needed to provide users with IDE autocomplete, typing, etc. They are
parsed in a single `_Options` class, which also provides a mechanism to
set default options on an App level, and override them on the more
granular API level.

This makes the long list of provided parameters a lot more manageable,
so I would like to use it for the `Jsonifier` as well, and re-add the
`debug` and `extra_files` arguments which I have dropped in previous
PRs. I'll submit a separate PR for this.

I renamed the `options` parameter to `swagger_ui_options` since it only
contains swagger UI options. This is a breaking change though, and we'll
need to highlight this upon release.

We still have quite a lot of `App`, `MiddlewareApp`, and abstract
classes. It would be great if we could find a way to reduce those
further, or at least find better naming to make it more clear what each
one does 🙂 .

Finally, I added examples on how the middleware can be used with third
party frameworks under `examples/frameworks`. Currently there's an
example for Starlette and Quart, but this should be easy to extend. They
also show how the `ASGIDecorator` and `StarletteDecorator` from my
previous PR can be used.

connexion/__init__.py

@@ -9,9 +9,8 @@
 
 import werkzeug.exceptions as exceptions  # NOQA
 
-from .apis import AbstractAPI  # NOQA
 from .apps import AbstractApp  # NOQA
-from .apps.async_app import AsyncApp
+from .apps.asynchronous import AsyncApp
 from .datastructures import NoContent  # NOQA
 from .exceptions import ProblemException  # NOQA
 from .problem import problem  # NOQA
@@ -21,14 +20,14 @@
 try:
     from flask import request  # NOQA
 
-    from connexion.apis.flask_api import FlaskApi  # NOQA
-    from connexion.apps.flask_app import FlaskApp
+    from connexion.apps.flask import FlaskApi, FlaskApp
 except ImportError as e:  # pragma: no cover
     _flask_not_installed_error = not_installed_error(e)
     FlaskApi = _flask_not_installed_error  # type: ignore
     FlaskApp = _flask_not_installed_error  # type: ignore
 
-from connexion.apps.async_app import AsyncApi, AsyncApp, ConnexionMiddleware
+from connexion.apps.asynchronous import AsyncApi, AsyncApp
+from connexion.middleware import ConnexionMiddleware
 
 App = FlaskApp
 Api = FlaskApi