Allow attesters/proposers to override value of `is_payload_data_available`

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.