Support explicit OPTIONS routes

[liborch]

Allows explicitly registered OPTIONS routes to handle Local Network Access (LNA) preflight requests.

Currently, the automatic OPTIONS fallback is used before a concrete OPTIONS route can be matched. With this change, explicit OPTIONS routes take precedence, while the existing automatic "/*" handling is preserved.

[gittiver]

I don't see a need for explicit OPTION routes, they are handled by the automatic fallback, there is no content in body, what shall be achieved here?

[liborch]

Standard CrowCpp handling does not make Chrome happy if you want access local network like WebSocket running on localhost. It requires return e.g. Access-Control-Allow-Private-Network: true.
This patch gives you a chance to write your own handler that will fulfill Chrome's needs and will be more flexible than CrowCpp release cycle.

[gittiver]

I would assume this is already possible via Cors Handler middleware.
see: https://github.com/CrowCpp/Crow/blob/master/examples/middlewares/example_cors.cpp