Feat/bill manager

[RafaelJohn9]

Description

I've added the BillManager API endpoints, with both unit and integration tests.

• Shouldn't be merged yet

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires documentation update
• Refactor (code structure improvements, no new functionality)
• Tests (addition or improvement of tests)
• Chore (changes to tooling, CI/CD, or metadata)

How Has This Been Tested?

Checklist

• My code follows the project's coding style guidelines
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (if applicable)
• My changes generate no new warnings or errors
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

Screenshots (if applicable)

Additional Context