Create a GraphQL mutation to create Jira issues for vulnerabilities in the Security tab of a pipeline

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Why are we doing this work?

In #441954 (comment 1826946980) it came to our attention that we do not have a way to link Vulnerabilities that are found in the Security tab of a pipeline (i.e Security::Findings) to Jira issues.

Implementation plan

1. Create Mutations::Security::Finding::CreateExternalIssue
2. Make sure it accepts a Security::Finding ID as an argument
3. Make sure it either creates a new Vulnerability or uses an existing one
4. Make sure it creates a Vulnerabilities::ExternalIssueLink record