Indexing a field to use in the default sort of the occurrence search api

[marcos-lg]

Our occurrence search api uses this sort as default when the q parameter is not sent:

"sort": [
        {
            "year": {
                "order": "desc"
            }
        },
        {
            "month": {
                "order": "asc"
            }
        },
        {
            "id": {
                "order": "asc"
            }
        }
    ]

Queries with no filter but this sort appear quite often in the ES slow query logs. Therefore, it's worth testing an alternative that index a field with a calculated value for the year-month-id combination and use it to sort in the occurrence search api.

@MortenHofft if HPs or other UIs use some other default sorting I can also include it.

[MortenHofft]

HPs curently do year, month, day, gbifId if there are no q param. I can adapt that to fit the API if needed