WS-575: image ingestion manifest

• additional_metadata is now AbstractTextFile instead of Path
• image ingestion manifest example is same as used elsewhere in ingest_envoy
• implemented more tests
• for consistency, now using enum values always for ScienceProductType and Telescope, rather than sometimes strings and sometimes enum values.

TODO in a subsequent MR:

• fix the creation of the manifest output group, which turns out not to be working yet