refactor: interfaced the billing service

[samuelIkoli]

Description

To improve code modularity, testability, and adherence to the dependency inversion principle, billing services where refactored to utilize interfaces.
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Related Issue (Link to Github issue)

#367
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How Has This Been Tested?

I ran automated and reliable test files that have been pre-written after creating a docker container to simulate the environment. Also used postman to test the endoinds
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Screenshots:

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Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Refactor
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Checklist:

• My code follows the code style of this project.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.