Add ItemResource class + Refactor Get Items API

[nijya]

1. adding a new item "Apple" with id 25 to shopcart with id 55, then use the newly added ItemSource Class to get it.
2. Successfully get it from the newly added ItemSource Class.

[codecov[bot]]

Codecov Report

Patch coverage: 44.44% and project coverage change: +0.42% :tada:

Comparison is base (e0b3ba7) 94.59% compared to head (014d782) 95.02%.

:exclamation: Current head 014d782 differs from pull request most recent head 4def964. Consider uploading reports for the commit 4def964 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #144 +/- ## ========================================== + Coverage 94.59% 95.02% +0.42% ========================================== Files 8 8 Lines 481 482 +1 ========================================== + Hits 455 458 +3 + Misses 26 24 -2 ``` | [Files Changed](https://app.codecov.io/gh/CSCI-GA-2820-SU23-001/shopcarts/pull/144?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CSCI-GA-2820-SU23-001) | Coverage Δ | | |---|---|---| | [service/routes.py](https://app.codecov.io/gh/CSCI-GA-2820-SU23-001/shopcarts/pull/144?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CSCI-GA-2820-SU23-001#diff-c2VydmljZS9yb3V0ZXMucHk=) | `92.00% <44.44%> (-0.37%)` | :arrow_down: | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/CSCI-GA-2820-SU23-001/shopcarts/pull/144/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CSCI-GA-2820-SU23-001)

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