remove middleware stack building to avoid conflict with exception handler

[vincentsarago]

closes #719
closes #720

This PR removes the middleware stack building and the add_middleware method to avoid conflict with the error handler middleware created by starlette later.

the middleware stack will be created by starlette on the first app call

[lossyrob]

@vincentsarago why use the internal user_middleware list here, and not just use add_middleware? Do we need to add middleware after a call is made to the FastAPI app, so you're trying to avoid an error with registering middleware after the initialization? That seems like it might not be a good pattern, to register middleware after initialization; if we aren't supporting that, thne self.app.add_middleware(middleware) seems better here.

[vincentsarago]

good call, let's use self.app.add_middleware

🙏 thanks @lossyrob

[vincentsarago]

ohhhh I see why we can't use starlette's add_middleware.

The add_middleware method expect a list of Callable + arguments (which will be turned into Middleware object) https://github.com/encode/starlette/blob/5a1bec33f8d6a669a3670f51034de83292d19408/starlette/applications.py#L134-L142

But the StacApi.middlewares is a list of Middleware objects so we need to insert them into the user_middleware list ourself.

That's said, I think it would be good to raise the same error from starlette https://github.com/encode/starlette/blob/5a1bec33f8d6a669a3670f51034de83292d19408/starlette/applications.py#L140-L141