Add ExistingResourcePolicy support to restore API

[shubham-pampattiwar]

Describe the problem/challenge you have

Velero currently does not support any restore policy on kubernetes resources that are already present in-cluster. Velero skips over the restore of the resource if it already exists in the namespace/cluster irrespective of whether the resource present in the restore is the same or different from the one present on the cluster. It is desired that Velero gives the option to the user to decide whether or not the resource in backup should overwrite the one present in the cluster.

Describe the solution you'd like
It is desired to add the functionality to specify whether or not to overwrite the instance of resource in cluster with the one present in backup during the restore process. This can be achieved via introduction of ExistingResourcePolicy to restore API for Kubernetes resources.

Anything else you would like to add:
Related issues: #4066

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[reasonerjt]

Reopening as this PR is not merged:
#4628