Allow datetime search to include start_datetime and end_datetime

[rhysrevans3]

Is your feature request related to a problem? Please describe.
If an item's datetime is not set i.e it's using a start_datetime and end_datetime. It's exclude from temporal searches.

Describe the solution you'd like
Start_datetime and end_datetime to be included in temporal searches.

Describe alternatives you've considered
An advanced datetime search that includes all temporal assets.

[StijnCaerts]

The current behaviour is indeed not conforming to the OGC API - Features specification.
https://docs.ogc.org/is/17-069r3/17-069r3.html#_parameter_datetime

If both the datetime and (start_datetime, end_datetime) are specified, it's up to the server implementation to decide which properties are considered. But if only one of these options is provided in the Item metadata, they should be respected by the query:

Only features that have a temporal geometry that intersects the temporal information in the datetime parameter SHALL be part of the result set, if the parameter is provided.

The current behaviour is also not correct in displaying no results at all when applying a datetime filter. If no temporal information is provided for a feature, it should not be excluded from the result.

The datetime parameter SHALL match all features in the collection that are not associated with a temporal geometry, too.

In practise, this cannot happen in STAC, because either one of these temporal properties is required by the STAC specification: https://github.com/radiantearth/stac-spec/blob/master/item-spec/item-spec.md#datetime

[StijnCaerts]

It might be interesting to look how this is handled in other implementations. It seems to be supported in Planetary Computer:
https://planetarycomputer.microsoft.com/api/stac/v1/search?datetime=2024-11-01T00:00:00.000Z/2024-11-16T00:00:00.000Z&collections=modis-13A1-061&limit=10