Add `exist_ok` attributes to repository, tag, other creatable lakeFS types

[nicholasjng]

The backend returning HTTP 409 errors (resource already exists) breaks idempotence in e.g. the tutorial notebook, which worsens UX for a docs development flow.

An easy fix is to allow their existence (e.g. by an exist_ok attribute like in pathlib), and return the object if it already exists.