Provide urls for Merge Requests and Issue links

This MR is about adding two urls (merge_requests_url and issues_url) to the Release API GET responses. These URLs will be then used in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/15764.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Style guides

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.