Consider moving "late init then update" functionality to client pkgs

[hasheddan]

What problem are you facing?

Currently, managed resource reconcilers late initialize spec parameters, then check for any differences with the current spec, and update the object if so.

Ref: https://github.com/crossplaneio/stack-gcp/pull/151#discussion_r376232888

This type of comparison usually lives in the client pkg rather than the controllers themselves.

How could Crossplane help solve your problem?

Move late initialization, then comparison to client pkgs.

Note: the actual update of the object's spec (i.e. the kube client call) should remain in the controller.