SignalR MessagePackHubProtocol should handle the EXTENSION value type.

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Is your feature request related to a problem? Please describe the problem.

I am using the signal R client to get data from a signal R server. Unfortunately, the MessagePackHubProtocol doesn't handle the EXTENSION value type.
It's commented out and it would be really useful uncommented.

### Describe the solution you'd like

Uncomment the code, and break to get out of the switch. Of course remove the RuntimeException throwing.
Adapt the relevant unit test MessagePackHubProtocolTest.

### Additional context

_No response_