Extending quote item with extension attributes - problem with load/save of attributes

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### Summary (*)

Recently, I was trying to extend quote item with extension attributes. I needed it inside cart and checkout. So there are actually two ways for loading and persisting extension attributes along extended model.

* First is creating plugins on repository of entity which has these new attributes.
For `CartItemRepository` this maybe would work for webapi - inside cart, checkout quote items are loaded in other ways - sometimes from `qoute` - by items collection, sometimes by directly loading model. It turns out that `CartItemRepository` is not very useful in frontend. Here comes also other grave problem - service contracts vs ,methods loading/returing/requiring concret implementations - many cases for `quote` and `quote item`.
* Second way is adding handles to `ExtensionPool`, but it requires resource model using `EntityManager`, which is is not recommended . Despite of this for qoute item it is imposible, beacause, crud operations are performed in 'old-fashioned' way.

Finally, to perform crud operations on cart and checkout, with main logic in subtotals I ended up with plugins for three classes and override 5 methods - `quote items` are not loaded in any consistent way.

### Examples (*)

Hard to provide - generally module doesn't seem to follow general guidelines from Magento.

### Proposed solution

Clean up crud operations for `quote` and `quote item`, so service contract are used more widely on frontend, so we are not relying on implementations, but on contracts.

Start by tracing CartItemRepository, ExtensionPool, and the quote and quote-item loading and persistence paths mentioned in the issue. Define the scope of consistent contract-based CRUD and verify that extension attributes load and save across cart and checkout flows without relying on concrete implementations.

Written by the indexing model from the issue text.