METS Upload: Read json summary when present

Instead of creating brand new elements each time, if the ALTO upload has been run before, the METS upload should use the IDs of these elements.

Read the values of key "elements" in the summary to get the mapping {<alto_id>: <arkindex_id>}.

When the JSON summary is provided, if an ALTO ID is missing, crash.

This summary is optional, keep the current behavior if absent.