Extend database model for storing sealed eCMRs

Related issues

Closes #56 (closed)

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

Extend database model for storing sealed eCMRs

* Add Entity class and Repository for sealed documents.
* Extend databaseChangeLog.
* Add dependency for mapping String to JSON column types.