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]

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 ⚠️

Additional details and impacted files

@@            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     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.