Add more links to _links node for releases in REST API

Background

In !107932 (comment 1223973278), it was found out that there's a discrepancy between the REST API and the GraphQL API for the links node.

In GraphQL, there are more URLs (*_mergeRequestsUrl and *_issuesUrl) that are exposed while they're not present on the REST API.

Proposal

Expose the *_merge_requests_url and *_issues_url under _links property so GraphQL and REST API will have parity.