Allow effects in `AuthenticatedRequest`

servant-client's `AuthenticatedRequest` is a newtype wrapping auth data as well as a way to modify a request:

```haskell
newtype AuthenticatedRequest a =
AuthenticatedRequest { unAuthReq :: (AuthClientData a, AuthClientData a -> Request -> Request ) }
```

This works well in many cases, but the `Request` modification function has to be pure, which is not enough in some cases (eg signing the `Request` requires a random nonce, or including a timestamp in the signature).

I've started exploring the following solution (`m` being destined to be ultimately bound with the `m` from `Client m`):

```haskell
newtype AuthenticatedRequest m a =
AuthenticatedRequest { unAuthReq :: (AuthClientData a, AuthClientData a -> Request -> m Request ) }
```

But I'm not sure if it's feasible to write the `HasClient` instance for `AuthProtect` in this context though.