add typing for assertions

[ships]

Description

Replaces t.unknown with a slightly more structured type for the Assertions. It is consistent with the jsonld specification - that the objects use item and status.

Related Issues

https://github.com/Docmaps-Project/docmaps/issues/23 (indirectly)

Checklist

• [x] I have tested these changes locally and they work as expected.
• [x] I have added or updated tests to cover any new functionality or bug fixes.
• [ ] I have updated the documentation to reflect any changes or additions to the project.
• [x] I have followed the project's code of conduct and conventions for commit messages.

Additional Information