Allow creating HttpsConnector with executor

[ebkalderon]

The hyper::client::HttpConnector that HttpsConnector uses under the hood has a new_with_executor() constructor. This is very useful for performance if the application already has an existing threadpool, allowing it to be reused instead of having the connector create one of its own, which as far as I can tell, is unfortunately hard-coded to only 4 threads. This constructor should really be exposed to ensure feature parity with hyper.