Moving some /rows shared utils
There are some classes, functions from /rows that will be used in https://github.com/huggingface/datasets-server/pull/1516 and https://github.com/huggingface/datasets-server/pull/1418, to avoid duplicate code, moving some of them to dedicated utils or to existing files.

Moving some /rows shared utils: There are some classes, functions from /rows that will be used in https://github.com/huggingface/datasets-server/pull/1516 and https://github.com/huggingface/datasets-server/pull/1418, to avoid duplicate code, moving some of them to dedicated utils or to existing files.
