HTTPMethod enum causes conflicts with ESPAsyncWebServer

[sidoh]

Thank you very much for porting this to ESP32!

In one of my projects, I'm using ESPAsyncWebServer, which also defines a non-class enum for HTTP_GET, etc. I wanted to use WiFiManager as well, but since that pulls in WebServer, I was getting conflicts for these constants.

I re-defined these constants in WebServer_tng using enum class, which resolved the issue. Changes are here:

sidoh@75d2645

Happy to open a PR, but since this affects consumers (they'd need to reference these constants using the HTTPMethod:: namespace), I figured an issue would be more appropriate.