Add event check-in system with on-chain verification

[robertocarlous]

Description

The on-chain check-in system is implemented and the check-in behavior is passing tests.
Fixes #115

Type of Change

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• Bug fix (non-breaking change which fixes an issue)

Changes Made

Testing

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• Test A
• Test B

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• 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

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Additional Notes

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