As system, I can set the assignee of Test Runs whenever using the REST API for importing Xray JSON results

As system, I can set the assignee of Test Runs whenever using the REST API for importing Xray JSON results as depicted in https://confluence.xpand-addons.com/display/XRAY/Import+Execution+Results#ImportExecutionResults-XrayJSONformat. 

For this, a new attribute "assignee" must be added to the "test" object.