Add embed support for AsChangeSet

[vnghia]

This is a copy verbatimly from Insertable. Since the interaction between treat_none_as_default_value and embed isn't allowed in Insertable so it is not allowed here as well.

Closes #1917

[weiznich]

Thanks for opening this PR.
Could you also add a test like that one to demonstrate that it works as expected?

Additionally we should document this attribute here. See the relevant documentation on the Insertable derive for an example

Finally that's a great candidate for adding an entry to our Changelog.md file.

[vnghia]

I think it's ready. Thank you for your pointers. It makes everything become much easier

[weiznich]

Thanks, looks good now 👍

(I just pushed a minor addition that checks as well that you cannot mix embed + serialize_as)