Allow attesters/proposers to override value of `is_payload_data_available`

[potuz]

In #4884 we added a new helper function is_payload_data_available that is used by attesters and proposers do decide between empty and full nodes for a given root. I believe attesters and proposers should be able to return True from this function, even if the PTC has voted for False, if they have the full data available and timely. One could argue that this should be done only if the PTC has achieved some minimum vote for True, even if it wasn't a majority.