Support overriding .meta:residual-child-node-category for auto-export

On a production system, newly created channels should be treated as content. However, working in a local development environment, adding a new channel should probably be exported as config. Therefore, the auto-export configuration should be extended to support overriding certain configuration model (default) categorization.

Another use case are the container items in an hst container, which should be exported as content rather than config.