[Refactor]: unify all notation to camelCase

[Istiopaxx]

Doing

• [Refactor]: improve creating entity logic #113
• Transfer all snake_case to camelCase
• Change test directory structure -> set all unit tests to test/unit folder, integration test(repo, e2e) to test/integration

TODO

• Add decorator for Rest API using class-transformer @Expose, @Exclude, and expose with snake_case for compatibility
• Fix tests not to test implementation, just test return value or assertion
• Devide domain object with entity
• Move domain requirements/logic to domain object

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