Expose more ways to obtain information about requests

**Is your feature request related to a problem? Please describe.**
In a reverse proxy application it is essential to know information about a request like its protocol (http, ws, etc.), or whether the request itself is a WebSocket.

Otherwise `drogon::app().forward(...)` can't make proper forwarding to the backend without rewriting what is already implemented in [HttpServer.cc](https://github.com/drogonframework/drogon/blob/96919df488e0ebaa0ed304bbd76bba33508df3cc/lib/src/HttpServer.cc#L1021).

**Describe the solution you'd like**
Several options:
1. Expose the protocol string.
2. Add a local boolean member within requests called `isWebSocket`.
3. Expose the already implemented method in [HttpServer.cc](https://github.com/drogonframework/drogon/blob/96919df488e0ebaa0ed304bbd76bba33508df3cc/lib/src/HttpServer.cc#L1021).

**Describe alternatives you've considered**
Rewriting the current implementation.