Ingest the value of the special key 'private' into the corresponding column

[drnextgis]

Fixes #243

This change enables the ingestion of the content from the private key into the private column of the database in hydrated mode (the default). For example, for a STAC item like:

{
    "id": "pgstac-test-item-0003",
    "private": {"created_by": "stac-task"},
    ...
}

The {"created_by": "stac-task"} json document will be ingested into the private column and will not appear in the content column.

Example:

postgis=# select private, content from items where id = 'pgstac-test-item-0003';
-[ RECORD 1 ]-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
private | {"created_by": "stac-task"}
content | {"bbox": [-85.4501, 36.1249, -85.4249, 36.1501], "links": [{"rel": "collection", "href": "joplin", "type": "application/json"}], "assets": {"cog": {"href": "s3://noaa-eri-pds/2020_Nashville_Tornado/20200307a_RGB/20200307aC0852700w360900n.tif", "type": "image/tiff; application=geotiff; profile=cloud-optimized"}}, "properties": {"event": "Nashville Tornado", "datetime": "2020-03-07T00:00:00Z"}, "stac_extensions": []}