add support to update attachments even when they are pending deletion

[AmitKumarDas]

UserStory: As an infrastructure admin, I want my workload specific un-installer to uninstall all native Kubernetes resources, custom resources as well related custom resource definitions. I would like to delete the workload specific namespace & expect all associated pods, deployments, crds, custom resources (even with finalizers) get deleted.

Metac ignores reconciliation if attachment resource(s) is pending deletion. This issue tackles this limitation of Metac by proposing a suitable enhancement.

[AmitKumarDas]

This PR (#60) was committed recently to handle updating attachment resources even if these are pending deletion. However, it will be better to have this tunable declared in the specs of meta controller (i.e. API enhancement). This API enhancement should work during sync as well as finalize hooks.