Add support to programmatically configure more complicated repository sources (e.g., federated) in DnaConfiguration

The DnaConfiguration allows setting up repository sources, but it doesn't provide a way to do anything but set properties on those sources. The DnaConfiguration should provide a way of adding configuration content underneath the node that represents the source. This would make it possible to programmatically repository sources that have more complicated configuration requirements.

A workaround is to define the configuration in a file and then import that file.