Allow apps to create and retrieve links between epics and stories via API in next-gen projects

Summary

Currently, there's no way to get the reference of epics and sub-issues (e.g stories). It would be great to have this feature as this allow vendors form the relationship between epics and it's sub-issues.

Use Cases:

1. When an app is subscribed to a webhook via the descriptor, there's no way to see which epic the story is linked to
2. When an app's feature is to create diagram for epics and sub-issues, there's no way to form it in next-gen projects