Reconcilers should use an interface for dataplane client instead of tightly coupling with dataplane.KongClient

[pmalek]

Problem statement

Currently most of the reconcilers use *dataplane.KongClient field for interaction with dataplane.

This creates a tight coupling and makes it hard to test.

#3759 introduced DataPlane interface (which can be moved to a more appropriate place if it's needed by non gateway reconcilers).

The proposal is to use this instead of concrete type *dataplane.KongClient to allow easier testing.