Add ParentIssueURL field to Issue struct

[SP3269]

Please add ParentIssueURL field to the Issue struct. This helps to easily identify issues that are child issues of other issues without using /parent API call.

The API spec description of the field:

          "parent_issue_url": {
            "description": "URL to get the parent issue of this issue, if it is a sub-issue",
            "type": "string",
            "format": "uri",
            "nullable": true
          },