Search datetime parameter has incorrect semantics for instant

[philvarner]

stac-pydantic/stac_pydantic/api/search.py

Line 68 in c2db77c

# Single date is interpreted as end date

The semantics for a Search datetime parameter that contains an instant (e.g., 2000-02-11T00:00:00.000Z) is that the Item.properties.datetime value should match that datetime parameter exactly. However, the code linked to above treats it as if the query were ../2000-02-11T00:00:00.000Z meaning all Items with datetimes prior should match, which is incorrect.