Add an option to fill default namespace in manifest if no other namespace set

[alexsomesan]

This change introduces a falg to enable filling the default namespace in the manifest when no namespace is specified. By default the flag is off and the tool behaves just like before this change.

The solution is not ideal because there doesn't seem to be a robust way to check if a specific resource type requires a namespace without actually making API calls to a cluster. However, there seem to be on average more resources requiring a namespace then not in a specific setup, so it's easier to add namespace everywhere and then remove it from the resources that don't need it.

Of course, if it turns out there is a way to formally identify namespaced resources offline, this feature should make use of that.