Searching by datetime doesn't work when specifying simple dates like `2023` or `2023-01-01/2023-12-31`

[alexgleith]

It does work when you do ISO datetimes, like 2023-01-01T00:00:00.000Z/2023-12-31T23:59:59.999Z.

[gadomski]

Changing this to enhancement because the "simplified" syntax (e.g. 2023 or 2023-01-01/2023-12-31) isn't part of the spec, it's just a convenience that was added to (e.g.) pystac-client: https://github.com/stac-utils/pystac-client/pull/33