text: Title: Would it be useful to have `req.get_header_as_int()`?
  Body: Sometimes it might be handy to easily get a header's value as `int`, where validation errors would automatically result in an HTTP 400, along the lines of  [`req.get_header_as_datetime`](https://falcon.readthedocs.io/en/stable/api/request_and_response_wsgi.html#falcon.Request.get_header_as_datetime) & [`req.get_param_as_int`](https://falcon.readthedocs.io/en/stable/api/request_and_response_wsgi.html#falcon.Request.get_param_as_int).