Simplify ChangeProcessor and EventHandler

[pleshakov]

Remove CaptureUpsertChange() and CaptureDeleteChange() from ChangeProcessor and pass all changes directly to
Process() instead. As a result, the clients will only need to call Process(), which will simplify them.
Now the clients make a combination of CaptureUpsertChange() and CaptureDeleteChange() calls followed by a call to
Process().

See https://github.com/nginxinc/nginx-kubernetes-gateway/blob/d121ff136620844826ae5d20024e876af6c99119/internal/state/change_processor.go#L181