Partial_row_log_event to Include Total Row Event Size

For the 12.3 preview release, MDEV-32570 had a shortcoming in which the Partial_row_log_event overestimates the amount of memory to allocate for the rebuilt Row_log_event on the replica. This is because the replica estimates the amount of memory to allocate based on the total number of fragments multiplied by the size of each fragment. The last fragment likely doesn't need the entire event allocated.

This can be optimized by including the total size of the event in the Partial_row_log_event header, so the replica knows to only allocate that amount of memory.