Export new object data from GitHub lag ingestion back into SWH main archive

The GitHub lag ingestion results in both new graph data and new objects. Both need to be imported into the SWH main archive. This issue tracks importing of objects (while #52 tracks importing of graph data).

The import of GitHub lag ingestion objects will happen in the content leaf nodes of the graph dataset that we want to export to the SWH main archive (see #52).

Looks like this is for all practical purposes the same as #52, as via the MR implementing this (!81) non-content graph nodes and content nodes (leaves) with the actual source code data will be exported at the same time, in the same files, via essentially the same mechanism.