Trying out AdditionalArguments class
Completed AdditionalArgument
compilable version
Completed Command Class without input validation
Put enum types into interfaces.

Enums should not be defined in implementations since they’re used by
other implementations which may not know each other due to the
abstraction constraints (but interfaces are visible to them).
Update Handler structures
Change Handler to abstract class

Fixed a few abstraction issues.
standardised to DBConnector instead of DbConnector