feat(engineioxide): specific transport restrictions (fix issue #49)

[Totodore]

This PR adds a field in the EngineIoConfig struct so the user is able to specify a limited number of transports, by default all transports are allowed).

For example, only websocket requests will be allowed, but polling request will be denied with a 400 Bad Request error.

let conf = EngineIoConfig::builder()
            .transports([TransportType::Websocket])
            .build();