**What**
There is actually an issue with using the `fields` query params with the way the repositories are using our custom query strategy. This pr aims to fix this issue by reworking the strategy.
What we had to do was to rework the way the selects are built for each subquery in order to follow the aliasing convention and to be taken into consideration. Alongside these changes, the join used to always select everything, this needed to be changed so that if there are any selects provided for a join, the join should not select everything and let the query select the fields that are requested.
Another notable change is that all the repositories are now using the repository util in order to centralize the customization and to have a single place to update when this kind of issue arises. This means that the eager relations when using the query builder are not necessarily taken into account. For that reason, I have removed the `shipping_option` eager option in favor of explicitly asking for the relations like we started to do it in some places.
FIXES CORE-1413
**What**
I have created a new method on the cart service which is `addLineItems`, allowing a user to add one or multiple items in an optimized way. Also updated the `generate` method from the line item service which now also accept a object data or a collection of data which. Various places have been optimized and cache support has been added to the price selection strategy.
The overall optimization allows to reach another 9000% improvement in the response time as a median (Creating a cart with 6 items):
| | Min (ms) | Median (ms) | Max (ms) | Median Improvement (%)
|---|:-:|---|---|---|
| Before optimisation | 1200 | 9999 | 12698 | N/A
| After optimisation | 63 | 252 | 500 | 39x
| After re optimisation | 56 | 82 | 399 | 121x
| After including addressed feedback | 65 | 202 | 495 | 49x
FIXES CORE-722
