Reduce code duplication in health assessment implementation

[alexmt]

Summary

After the reflection-based conversion was deprecated in K8S v1.19 the health assessment contains a lot of duplicated code ( see https://github.com/argoproj/gitops-engine/pull/139 ). We should consider using unstructured package directly to reduce code duplication.

Additionally we should try to use https://github.com/kubernetes/kubectl/blob/master/pkg/polymorphichelpers/rollout_status.go get rid of some copied code.