add update total price func and add related test cases

[carolinetfls]

add the total price method in the shop_cart model add the call to total price method in the routes code when an item is added, deleted or updated to a shopcart

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 87.50000% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 95.18%. Comparing base (c33d022) to head (d2a40f8). Report is 3 commits behind head on master.

Files	Patch %	Lines
service/routes.py	77.77%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #59 +/- ## ========================================== - Coverage 95.50% 95.18% -0.32% ========================================== Files 11 11 Lines 378 395 +17 ========================================== + Hits 361 376 +15 - Misses 17 19 +2 ```

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