add polling timestamp to masu sources endpoint

[maskarb]

Description

This change will add polling timestamp to masu sources endpoint.

Testing

1. visit http://localhost:5042/api/cost-management/v1/sources and see polling-timestamp for provider.

Release Notes

• proposed release note

* add polling timestamp to masu sources endpoint

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.1%. Comparing base (ee07f58) to head (3d06d42).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #5437     +/-   ##
=======================================
- Coverage   94.2%   94.1%   -0.0%     
=======================================
  Files        370     370             
  Lines      31550   31551      +1     
  Branches    3381    3381             
=======================================
- Hits       29706   29705      -1     
- Misses      1196    1199      +3     
+ Partials     648     647      -1