Add REST API endpoint to list related epics

Related to #351597 (closed).

We need a new REST endpoint (similar to the one used by Issue Links API) to list related epics.

We already have API resources that use the term links and are reserved for parent-child epic hierarchy, so we need to use different naming matching the other resources for this feature (TBC, see #351597 (comment 837691210)).

Example endpoint

GET /api/:version/groups/:id/epics/:epic_iid/related_epics

This page may contain information related to upcoming products, features and functionality. It is important to note that the information presented is for informational purposes only, so please do not rely on the information for purchasing or planning purposes. Just like with all projects, the items mentioned on the page are subject to change or delay, and the development, release, and timing of any products, features, or functionality remain at the sole discretion of GitLab Inc. This feature would be developed behind a feature flag and needs to perform license checks (implemented in #352439 (closed))

This page may contain information related to upcoming products, features and functionality. It is important to note that the information presented is for informational purposes only, so please do not rely on the information for purchasing or planning purposes. Just like with all projects, the items mentioned on the page are subject to change or delay, and the development, release, and timing of any products, features, or functionality remain at the sole discretion of GitLab Inc.