Support of special attribute updates in add_property

Summary

The Entity.add_property method now checks whether the name of an added property matches a special attribute and, if so, updates the attribute instead.
Addresses #217

Focus

• Should similar behaviour be supported for updates using only id? If so, how would I go about finding out which id the name attribute/property has server-side?
• As the method is called add_property and special attributes do not support multi-properties (as far as I know?), should we guard against an overwrite by only setting the attribute if it is currently None?

Test Environment

A unit test is included, is it sufficient?

Check List for the Author

• All automated tests pass
• Reference related issues
• Up-to-date CHANGELOG.md (or not necessary)
• Up-to-date JSON schema (or not necessary)
• Appropriate user and developer documentation (or not necessary)
• Annotations in code (Gitlab comments)
  • Intent of new code
  • Problems with old code
  • Why this implementation?

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)
• The test environment setup works and the intended behavior is reproducible in the test environment
• In-code documentation and comments are up-to-date.
• Check: Are there specifications? Are they satisfied?