Allow usage of RawDataChannel during EngineConnectionState.Intent

[aromaa]

In proxied environment its sometimes useful to allow the communication between the proxy and the backend server before any authentication has begun. For example the proxy can be used to some degree as "source of truth" where it can provide information for sync points, capabilities and other relevant information that plugins might want to process before the ServerSideConnectionEvent.Auth begin.

While intercommunication trough messaging services should be preferred, sometimes the delay can cause race conditions and the proxy could be used to retrieve the more relevant up to date information on such cases.

This PR allows the following capability by allowing plugin developers to use the RawDataChannel during ServerSideConnectionEvent.Intent and delays the authentication process until no more requests are in-flight. By restricting this to only raw data channels, we can fill this niche use case without more evasive changes to the login process.

Open question:

• Do we want to model this more explicitly in the API? Or update the documentation and call it a day.