Replace TestRunStatus custom field value storage from Jira API into an Xray-owned structure

Bug in Jira:

https://jira.atlassian.com/browse/JRASERVER-12796

Jira may write more than one value for a custom field.

Jira's solution is only a workaround. When the value is accessed, if more than one value is found, a warning message is logged and one of the values is picked.

This may occur when execution results are updated concurrently or in very quick succession since Xray will trigger a 'write' action on the custom field.

This can lead to the wrong TestRunStatus being stored.

The proposed solution is to remove Xray's reliance on the Jira Custom Field API for data storage and move this process into Xray-owned data storage API.