We want to support IO functionality in DocVec such as from/to json, csv, bytes, etc. For example, to/from json is needed to make it FastAPI-compatible.

However, we can not use the existing IOMixin here as the logic behind DocVec is different from DocList and we'd have to overwrite some functions.

Solution:
Create an abstract mixin containing functions that the two classes share, and for other functions create separate mixins inheriting from the abstract class