Embed Starscream 3

[FZambia]

Relates #48

Looking at Starscream issue tracker migration to StarScream 4 can result in some connection problems. This requires a bit more consideration in general. Maybe there is a better alternative these days?

Since we already have dependency conflicts mentioned in #48 I think for now we can embed Starscream 3 to SwiftCentrifuge.

Removed all public and open access level identifiers.

[fedulvtubudul]

There is an official WebSocket implementation from Apple https://developer.apple.com/documentation/foundation/urlsessionwebsockettask. Possible reason to avoid it for now is that it is iOS 13+ compatible, while centrifuge-swift is declared to support iOS 9+.

So in ideal world I would suggest isolating WebSockets transport into an abstraction with several possible implementations like Starscream 3 for those who needs older systems support or URLSession's WebSockets for those who are ready to drop everything below iOS 13.

[FZambia]

Yeah, I think moving to the official WS implementation is a good thing. Starscream 4 already does this internally - so I'd say we can use it eventually, but need to research a bit. Maybe there is an actively developed fork of it.