add example that showcases `defaulting` controller

[AmitKumarDas]

UseCase: As a developer who uses metac to build my controller, I need metac to support setting of defaults against my watched resource specifications. These defaults should be set against the fields of the resource that are not set.

NOTE: This controller is expected support GitOps requirements. In other words the specifications without defaults are stored in my git repository. However, the resource in etcd has the required defaults set. There should not be any loss of values due to differences from my git version versus the etcd version.

Solution: Metac as such does not need any changes to support this requirement. We need to verify by adding following items:

• add example(s) that showcases above usecase
• add integration test(s) that showcases above usecase

[AmitKumarDas]

We can try below metacontrollers to prove if Metac supports this usecase:

• DecoratorController
• GenericController