mix bonny.gen.manifest add an override callback

[mruoss]

Add a callback to mix bonny.gen.manifest that lets users override the generated resources.

Example

def override_resource(%{"apiVersion" => "rbac.authorization.k8s.io/v1", "kind" => "ClusterRole"}), 
  do: #overrides

def override_resource(%{"apiVersion" => "v1", "kind" => "ServiceAccount"}),
  do: #overrides