DatetimeInterval pydantic model doesn't support open-ended date ranges

[phil-osk]

Per the the opportunity spec and order spec, dateinterval should support open-ended date ranges where .. represents an open end (e.g., ../20250101, 20250101/..) as specced in RFC 3339.

Pydantic model is:
DatetimeInterval = Annotated[ tuple[AwareDatetime, AwareDatetime], BeforeValidator(validate_before), AfterValidator(validate_after), WrapSerializer(serialize, return_type=str), WithJsonSchema({"type": "string"}, mode="serialization"), ]

AwareDateTime does not allow for None or other representation that an item in the tuple does not exist.

[gadomski]

Closing as duplicate of #35