Currently, in Jooby 4.x, WebSocketMessage does not provide a public API to access binary messages.
I need to handle binary WebSocket frames in my application, but there is no suitable
method to get the raw bytes or ByteBuffer.

This PR proposes adding a method such as getBytes() or getByteBuffer() to
WebSocketMessage to allow safe and convenient access to binary payloads,
without relying on internal classes.