Resolve "Add SealedDocument to data model"

Related issues

Add SealedDocument to data model.

• Includes: SealedeCMR, Seal and an optional precedingSeal.
• Increase version to 0.3.1

Acceptance criteria

• Implemented code changes don't diverge from the related issue.
• Documentation reflects the status quo after merge.
• Version number is incremented according to SemVer. (Snapshot versions are not merged/released.)
• Only released versions of dependencies are used (i.e. no snapshot versions).
• Pipeline still works.
• GitLab issue is up-to-date.

If necessary, the following sections have been updated:

• Swagger
• Readme.md
• Documentation

Proposed squash commit message

Add SealedDocument to data model.

* Includes: SealedeCMR, Seal and an optional precedingSeal.
* Increase version to 0.3.1

Co-authored-by: Lisa Gebel <lisa.gebel@iml.fraunhofer.de>

Closes #23 (closed)