[Feature Request] Add sdDatePublished to RO-Crate Export

[floWetzels]

The RO-Crate export currently always writes a datePublished entry for the root data entity, as required by the RO-Crate specification. This is an actual publication data, if one exists, or the creation date of the RO-Crate json file otherwise. In case that an actual publication date exists, the creation date should be annotated via the entry sdDatePublished.