ServerAPI: Add method to add new attribute

[tadeas-hejnic]

Is there an existing issue for this?

• I have searched the existing issues and added correct labels.

Please describe the feature you have in mind and explain what the current shortcomings are?

Method for adding new attribute is missing in ServerAPI class.

Tests of operations with attributes can not be implemented without this method.

Suggested implementation?

No response

Describe alternatives you've considered:

No response