Added Search Shopcart by User_id

[pr-2310]

Made the following changes

• Added search shopcart by user_id endpoint in routes.py
• Added test cases for search shopcart by user_id
• Added find by user_id method

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.68%. Comparing base (0cc7108) to head (fb4c4ad).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #66 +/- ## ========================================== + Coverage 95.56% 95.68% +0.12% ========================================== Files 11 11 Lines 428 440 +12 ========================================== + Hits 409 421 +12 Misses 19 19 ```

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