Add restriction on the namespace where App instances are created

[devdattakulkarni]

We should restrict app instances creation to the same namespace where KubePlus is installed.

Currently we have this restriction for creating ResourceComposition instances

We should add this check for app instance creation as well.

[devdattakulkarni]

The check will need to be added to the handleCustomAPIs method

[devdattakulkarni]

Actually, this is not an issue. KubePlus only intercepts the calls for creating application instances for the objects that are defined in the same namespace where KubePlus is running. This is achieved by the MutatingWebhookConfigration definition, where we have defined a Namespace selector, which only intercepts objects in the namespace that has a label "managedby: kubeplus". This label is put on the Namespace in which KubePlus is installed here.

So if we create an application object in another namespace, the call does not get intercepted. It will be good to clearly identify this in the documentation.