Add async Flask OAuth client #818

Is your feature request related to a problem? Please describe.

The Flask OAuth client uses Requests and is thus synchronous. Although Flask is a predominately a synchronous web framework, it does support asynchronous views. This raises the possibility of an async Flask view wanting to use an async OAuth client.

This may also apply to the Django OAuth client.

Describe the solution you'd like

An async variant of FlaskOAuth2App that uses HTTPX.

Describe alternatives you've considered

Current workaround is to create a custom OAuth app:

class AsyncFlaskOAuth2App(AsyncOAuth2Mixin, AsyncOpenIDMixin, BaseApp):
    client_cls = AsyncOAuth2Client

oauth = OAuth(app)
oauth.register(
    client_cls=AsyncFlaskOAuth2App,
    ...
)

Additional context

N/A