Add event_types_supported to the SSF transmitter metadata

[vivshankar]

Proposal

1. Add event_types_supported to the SSF transmitter metadata to indicate the available event types for receivers to request. This potentially allows the receiver to not need to perform additional actions on its end based on the stream config response. This is proposed as an optional property for backward compatibility.
2. Allow event types other than those listed in the event_types_supported to continue to be requested and used. This would cover event types that transmitters make available to specific receivers or for "beta" purposes.