Add flag and cuid getters to Entity

Summary

Added new protected property getters to Entity.
Needed for caosdb-advanced-user-tools!126 (merged)

Test Environment

No testing should be necessary, access should be read-only - does the dict always have strings as values?

Check List for the Author

• All automated tests pass
• Reference related issues
• Up-to-date CHANGELOG.md
• Appropriate user and developer documentation (or not necessary)
• Annotations in code (Gitlab comments)

Check List for the Reviewer

• I understand the intent of this MR
• All automated tests pass
• Up-to-date CHANGELOG.md (or not necessary)
• Appropriate user and developer documentation (or not necessary)
• In-code documentation and comments are up-to-date.