OnboardingServiceImpl needs to implement SignatureService

[oliverrahner]

Describe the bug
When the SignatureService interface was created, necessary refactoring for com.dke.data.agrirouter.impl.onboard.secured.OnboardServiceImpl had not been done.
This leads to duplicate code:
https://github.com/DKE-Data/agrirouter-api-java/blob/develop/agrirouter-api-java-impl/src/main/java/com/dke/data/agrirouter/impl/onboard/secured/OnboardingServiceImpl.java#L117-L160

Expected behavior
Code respects DRY, OnboardServiceImpl implements SignatureService