Ignore some fields when dumping with --data

--signature
--last_modified
--id

Actually we might also want to give an explicit list of fields we want to dump in the collection.

Start by locating the CLI entry point and the code that handles dumping with --data. Check how collection fields are currently selected, then define the expected behavior for ignoring --signature, --last_modified, and --id and for optionally supplying an explicit field list; done means the dump output follows those rules and the relevant tests pass.

Written by the indexing model from the issue text.