Crawl security issues for security merge requests

This adds ReleaseTools::Security::IssueCrawler, which can be used to find all security issues and merge requests given a root security issue such as https://gitlab.com/gitlab-org/gitlab/issues/194123.

This can then (in a future commit) be used to validate security merge requests per security issue, instead of validating them individually. This in turn allows us to prevent merging of some merge requests while other related merge requests are still being worked on.

This MR is one of the steps needed for https://gitlab.com/gitlab-org/release-tools/issues/364, and for https://gitlab.com/gitlab-org/release-tools/issues/369.