Added index_ref field to RNTuple serving as row index reference

Adding row indexing to RNTuple so Event augmentation and SharedWriter MP jobs are possible also in this format.
Also changed index synchronization API to use uint64_t instead of long long. Should have no effect on the current TTree format so no need for very deep review.

(lots of apparent code changes here because rebasing to the current main was really messy and I had to copy blocks by hand - but it's mainly difference in formatting)