Add vulnerabilityIssueLinkCreate GraphQL mutation

Why are we doing this work

Based on #267582[design_1641499459303.png] we need the ability to link multiple Vulnerabilities with an existing Issue

Implementation plan

• Create a service object that:
  • accepts an Issue and a list of Vulnerabilities
  • creates Vulnerabilities::IssueLink records for the selected Vulnerabilities and a given Issue with link_type of related
  • returns created Vulnerabilities::IssueLink records
• Create a GraphQL mutation that:
  • accepts a list of Vulnerabilities and an Issue
  • uses the service object to create necessary associations
  • sets query complexity according to snippet from #373816 (comment 1117079824)
• Consider firing a background worker if more than 100 (can be higher) Vulnerabilities are selected to prevent timeouts. This can be done in a follow-up issue. -> split off in #408365

Verification

You can use this test project

• Go to https://gitlab.com/gitlab-org/secure/tests/verify-373816/-/security/vulnerability_report
• Note down the ID of a vulnerability that does not have an issue link (the Activity column is empty).
• Go to https://gitlab.com/-/graphql-explorer and run this query:

  mutation($input: VulnerabilityIssueLinkCreateInput!) {
    vulnerabilityIssueLinkCreate(input: $input) {
      issueLinks {
        id
        issue {
          title
        }
        linkType
      }
    }
  }

• variables:

  {
    "input": {
      "clientMutationId": "xyz",
      "issueId": "gid://gitlab/Issue/127078651",
      "vulnerabilityIds": [
        "gid://gitlab/Vulnerability/<insert vuln id>"
      ]
    }
  }

• Reload the 'Vulnerability Report' page and you should see the vulnerability is now linked to issue 1 on the project.