Improve spec.initProvider to spec.forProvider merge

[lsviben]

Introduced with #237
When management policies are enabled, we are using the spec.initProvider field, and merging it to spec.forProvider. In addition,
we are calculating the fields which are part of spec.initProvider but not spec.forProvider and setting those fields into ignore_changes lifecycle hooks.

The issue here is that we are using mergo in a way that spec.initProvider fields override spec.forProvder fields:

// TODO(lsviben): Currently initProvider fields override forProvider
// fields if set. It should be the other way around.
// merge the initProvider and forProvider parameters
err = mergo.Merge(&params, initParams, mergo.WithSliceDeepCopy)
if err != nil {
	return nil, errors.Wrap(err, "cannot merge init and forProvider parameters")
}

This is due to using mergo.WithSliceDeepCopy which is on the other hand needed to merge arrays.

Ideally, spec.forProvider fields would take precedence over the spec.initProvider ones. Maybe there is an option with mergo to do something like that, or it wont be difficult to write ourselves.

But, as we don't recommenced or expect the same fields to be set, we could also error out if they set the same fields.