Starlette Integration Improvements - Should I PR?

[RamiAwar]

Hello @lepture ! I was using authlib in a work project recently, and I found it hard to do some things. One of which is revoking tokens.

1. So I extended the StarletteOAuth2App class into this, adding a revoke method. Would you be open to adding something like this to the official implementation?

2. Also, separate point but I can also to dedicate work time to it, but are you open to adding some caching on top of the load_server_metadata method? Some basic LRU cache + timer maybe so that we don't have to make an extra network call EVERY time we call this method. It probably does not change at all in most cases.

class CustomStarletteOAuth2App(StarletteOAuth2App):
    async def revoke_token(self, request: Request) -> None:
        token = request.session.get("token")
        # Nothing to do if no token
        if token is None or token.get("access_token") is None:
            return None

        metadata = await self.load_server_metadata()
        revocation_endpoint = metadata.get("revocation_endpoint")
        if revocation_endpoint is None:
            raise ValueError("Revocation endpoint not found in server metadata")

        async with self._get_oauth_client(**metadata) as client:
            client = cast(AsyncOAuth2Client, client)
            await client.revoke_token(url=revocation_endpoint, token=token.get("access_token"))
        return None