Item output is missing start_datetime and end_datetime

**Describe the bug**

If an input STAC item has `start_datetime` and `end_datetime` properties, like:
```
"properties": {
"datetime": "2011-01-01T00:00:00Z",
"description": "BCSD surface water projections hydrologic data",
"end_datetime": "2064-01-16T00:00:00",
"start_datetime": "2064-01-16T00:00:00",
"title": "sresb1.ukmo_hadcm3.1.monthly.swe.2064",
"units": "mm"
},
```

When requesting the item from Franklin, the start/end are missing:
```
{
"properties": {
"units": "mm",
"datetime": "2011-01-01T00:00:00Z",
"title": "sresb1.ukmo_hadcm3.1.monthly.swe.2064",
"description": "BCSD surface water projections hydrologic data"
}
```

**Expected behavior**
I think start_datetime and end_datetime should be in the item STAC shown in Franklin.

Start by reproducing the Franklin item request with an input STAC item containing start_datetime and end_datetime, then trace how item properties are assembled in the API response. Done means the returned STAC item preserves both properties, with a regression test covering the example.

Written by the indexing model from the issue text.