Feature/implement s2s calls

[jo-desa]

All Submissions:

• Have you followed the KelkooGroup Sales tracking Contributing guideline?
• Does your code follow the WordPress' coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Closes # https://kelkoogroup.atlassian.net/browse/TT-1140.

• Server To Server implementation added
• Unit test added
• Github actions workflow added

How to test the changes in this Pull Request:

1. Check https://github.com/KelkooGroup/woocommerce-kelkoogroup-salestracking/actions green

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully run tests with your changes locally?

Changelog entry

Enter a summary of all changes on this Pull Request. This will appear in the changelog if accepted.