add `datetime` property at links of the monthly level catalog.json

[JinIgarashi]

Please add datetime property at links if a link is item. this can help client to get datetime information without fetching further item.json again

https://undpgeohub.blob.core.windows.net/stacdata/nighttime-lights/2023/09/catalog.json

{
   "type":"Catalog",
   "id":"nighttime-lights-2023-09",
   "stac_version":"1.0.0",
   "description":"VIIRS DNB nighttime lights nightly mosaics in 2023-09",
   "links":[
      {
         "rel":"root",
         "href":"../../../catalog.json",
         "type":"application/json",
         "title":"UNDP STAC repository"
      },
      {
         "rel":"item",
         "href":"./11/SVDNB_npp_d20230911.json",
         "type":"application/json",
         "datetime": "2023-09-11T00:00:00Z" <- you can add datetime property here.
      },
      {
         "rel":"item",
         "href":"./12/SVDNB_npp_d20230912.json",
         "type":"application/json"
      },
      {
         "rel":"item",
         "href":"./09/SVDNB_npp_d20230909.json",
         "type":"application/json"
      },
      {
         "rel":"item",
         "href":"./10/SVDNB_npp_d20230910.json",
         "type":"application/json"
      },
      {
         "rel":"parent",
         "href":"../catalog.json",
         "type":"application/json",
         "title":"Nighttime lights in 2023"
      }
   ],
   "title":"Nighttime lights in 2023-09"
}

[iferencik]

closed by #11