`image-datetime` output format

[GermanHydrogen]

The default format for image-datetime is defined as "%Y-%m-%d %H:%M:%S.%f" but is currently "%Y-%m-%dT%H:%M:%S".
You can reproduce this issue with:

from datetime import datetime
from ifdo import ImageData

data = ImageData(image_datetime=datetime.now())
data.model_dump(mode="json", exclude_none=True)

Pydantics custom type system could be used to change this. In the future we might want to look into how to incorporate the image-datetime-format into the validation and serialization.