Introduce FF for `Bulk attach a vulnerability to an existing Jira issue`
As we are doing Enhanced Bulk Actions for the Vulnerability Report
in 3 iterations. We are using separate FF for each action:
- Bulk attach a vulnerability to an existing issue
- Bulk attach to a vulnerability to a new issue
- Bulk attach a vulnerability to an existing Jira issue
This issue will introduce FF for Bulk attach a vulnerability to an _existing_ Jira issue
so that both frontend and backend can use this during development work.