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.

CHANGELOG.md

@@ -10,6 +10,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Added ###
 
 * New setup extra `test` which installs the dependencies for testing.
+* The `Entity` properties `_cuid` and `_flags` are now available for read-only access
+  as `cuid` and `flags`, respectively.
 
 ### Changed ###