Save Alto IDs

Refs https://redmine.teklia.com/issues/2909

Each Alto element leading to the creation of an Arkindex element should have a metadata (name Alto ID, type reference) with the value of its ID attribute (when it's available of course).

Similarly, we need a new CLI option --json-summary (boolean, defaults to False), which will build a JSON file with the following structure:

{
   "alto_file": "ocr_xxx.xml",
   "arkindex_api_url": "https://demo.arkindex.org",
   "elements": {
     "ID_1234": "<UUID1>",
   }

}

This would allow us to find the relevant arkindex elements when looking up an Alto ID, and thus reuse this element.

In the end, we should be able to find the Arkindex ID of an Alto element from the json file or its metadatas.