[cleanup] [client/k8s.go] Merge similar functionality into one decent function

[vharsh]

Problem statement

Currently the client/k8s.go file has nearly 530 LoC, a lot of which is just duplicated functionality or slightly similar functionality, e.g. GetCStorVolumeAttachmentMap(), GetCVA(string), etc.
There's a general trend in this file where-in, a resource has a wrapper to

• GET a single resource of a particular KIND
• LIST all the resources of a particular KIND
• LIST all resource and return a map of them by some criteria, viz resource-name ➝ resource, or resource-label-key ➝ resource
  All of this is helpful in different use-cases, but it sort of pollutes the method this package is exposing and it'll likely keep increasing as support for more features are added, for example client package could have methods such as GetPV, GetPVs, GetPVMap, GetAllCStorPV. If a new resource is added(e.g. CV), the problem will get worse as now, there might be 4 more methods per resource(assuming same or similar requirements)

Potential solution(s)

Low Hanging fruit

• It might help crunch the LoC by merging some of GET & LIST functionality into one method, e.g. GetPV(pvName string) (*corev1.PersistentVolume, error) & GetPV() (*corev1.PersistentVolumeList, error) & GetPVs(pvNames []string) (*corev1.PersistentVolumeList, error) into ONE ListPV(vols []string) (*corev1.PersistentVolumeList, error)
• The newer one can perform a
  • GET, if the vols list has exactly one element,
  • A LIST if vols list is nil
  • A LIST & filter out the vols PVs if a set of one or more volumes are specified

NOTE: Need to double-check if there'd be any possible dead-ends with this refactor, specially during the fetch of namespaced resources.

[vharsh]

Closing this, as this is done.