all: implement partial AppConfig validations depending on its usage by Stark services

[christophercr]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report  
[X] Feature request
[ ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead see https://github.com/NationalBankBelgium/stark/blob/master/CONTRIBUTING.md#got-a-question-or-problem

Current behavior

Some services in the different Stark modules need the AppConfig entity to be provided but this is not ensured by any means.

Expected behavior

Some logic in the service constructor should ensure that the injected AppConfig for the different services is present and valid before such services are initiliazed.

What is the motivation / use case for changing the behavior?

Due to the modularity we want to achieve in Stark, every module should take care of validating the piece of the AppConfig it cares about.