feat: Return convenient errors to middlewares

[Rilord]

Context

For *requests_total Counter metric we need to track HTTP response status code and operation name inside middleware.

With convenient_errors: true (or "on") is enabled, ogen provides convenient way to map errors to status codes, however the problem is that these status-code-aware errors do not seem to be returned to the middleware chain .

Current Behavior

...
resp, err := next(req) 
...

err = errors.New("not found")

nothing can be done here.

Expected

...
resp, err := next(req) 
...

err = ConvenientErrorWithStatusCode{
  StatusCode: 404,
...
}

using errors.As it's possible to extract status code from the error.