Allow other httpd modules to implement WebSocketHandlers

[jchampio]

The architecture is currently a hub/spoke plugin model, but other server modules should be able to implement their own handlers using the same API.

Use mod_grpcbackend as the driving use case. #32 is one implementation, using an OPTIONAL_HOOK; is there another hook option that makes clients' lives easier?

[Thalhammer]

I doubt you can make it much easier for plugins than using an optional hook. Anyone writing Apache plugins needs to know how to use hooks anyway as they will need it for the plugin itself and they are pretty easy to use if you are already in the apache ecosystem.

PS: Its nice to see this project is not abandoned after all.