Added Unit tests, Major Refactors, Simplified Code Readability

[Andreas-Kreouzos]

This pull request contains major improvements and changes in the whole project:

1. Made package refactors for better readability
2. Added custom annotation for the phone number field, to check that it starts from 00 followed by the country code
3. Added more validation annotations to check user input in UserCreationRequest class
4. Removed duplicated code and unused classes
5. Revamped the S3 Bucket logic and used the proper AWS SDK version
6. Removed unnecessary dependencies from pom.xml
7. Introduced various unit test cases that cover almost the whole application