Adds a method for the cve_issue

What does this MR do and why?

Implements a new method into the Security::ImplementationIssue to return the respective Security::CveIssue. To search the Cve issue a finder class was created to:

1. First fetch the associated canonical issue.
2. Then, search for a cve issue across the security and canonical issues. 1. If one is found on security then it is retrieved, if not, then the search continues to canonical.
3. If a cve_issue is found on canonical, then it is returned.
4. If no issue is found on security or canonical nothing is retrieved.

The cve_issue created on the Security::ImplementationIssue calls the finder and returns an instance of Security::CvesIssue.

Related to gitlab-com/gl-infra/delivery#19710 (closed)

Testing

I performed a manual test with the security issues associated with the tracking issue. Ids were modified to prevent leaking any confidential details:

[1] pry(main)> issues = ReleaseTools::Security::IssueCrawler.new.related_security_issues
=> [#<ReleaseTools::Security::ImplementationIssue:0x0000000111782b90
  @iid=111,
  ]
[2] pry(main)> issues.count
=> 3
[3] pry(main)> cve_issues = []
=> []
[4] pry(main)> issues.each do |issue|
[4] pry(main)*   cve = issue.cves_issue
[4] pry(main)*   cve_issues << cve if cve.present?
[4] pry(main)* end
....
[5] pry(main)> cve_issues.count
=> 3

Author Check-list

• [-] Has documentation been updated?