Refactor Converter class in ruleset2dmn so it is unit testable

Refactor the class Converter [1] so it is unit testable. Currently it contains a lot of private static classes, that cannot be tested separately.

Acceptance criteria:

• Each method in the class is tested by a unit test.

[1] https://github.com/kiegroup/drools/blob/main/kie-dmn/kie-dmn-ruleset2dmn-parent/kie-dmn-ruleset2dmn/src/main/java/org/kie/dmn/ruleset2dmn/Converter.java