This is called on all sources before recording is started. This allows a source to return a list of new sources that should be added to the recording. This is useful for abstract sources (such as "Player" or "World Settings") which are convenience wrappers for existing sources (such as a Actor Source). In these cases, these abstract sources simply do their logic to find out which new sources need to be made and then return them. These new sources only need to exist for the lifespan of a single UTakeRecorderSources recording.