Add security finding external issue link create mutation

What does this MR do and why?

Add security finding external issue link create mutation

This adds a mutation to allow creating an external issue link for a security finding. It finds the or creates a related vulnerability and then creates the external issue link.

References

Related #441954 (closed)

Screenshots or screen recordings

create-jira-issue

How to set up and validate locally

1. Enable feature flag http://gdk.test:3000/rails/features/finding_create_jira_issue_mutation
2. Set up Jira integration in a new project with new findings appearing in an MR
3. Click on a finding in the MR so the modal opens and click "Create Jira issue"
4. Validate it triggers the security finding mutation to create an external issue link
5. Validate it redirects to your Jira ticket (this can take up to more than a minute)

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.