Add REST API endpoints to create and delete related epics

Related to #351597 (closed).

We need REST endpoints (similar to the ones used by Issue Links API) to create and delete related epics.

Example endpoints

POST /api/:version/groups/:id/epics/:epic_iid/related_epics
DELETE /api/:version/groups/:id/epics/:epic_iid/related_epics/:related_epic_id

This feature would be developed behind a feature flag and needs to perform license checks (implemented in #352439 (closed))