Improve user experience around provider-specific middlewares

[fselmo]

What was wrong?

• Currently there is a middleware onion on the RequestManager and there are provider-specific middlewares. This leads to confusion like in Option for turning off HTTPRequestRetry #2770. We should re-think the way that request retry is handled in a more user-friendly way.

How can it be fixed?

One possible solution is to remove the idea of middlewares on providers and leave it to the middleware onion only. If we want to add some provider-specific features, these might be nice features as flags on the provider instead, or as provider-specific middleware that may be added to the onion itself in the RequestManager class.

Suggestions / opinions encouraged!