Epics as a list option for Issue Boards
### Problem to solve
surfaced in https://gitlab.com/gitlab-org/gitlab/-/issues/207899#note_308837120
> *As a User of issue boards,*
>
> *I want to add lists to my issue board that represent epics,*
>
> *So i can map issues to epics easier, and vizualize my issues within each epic*
### Intended users
* [Parker (Product Manager)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#parker-product-manager)
* [Delaney (Development Team Lead)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#delaney-development-team-lead)
### Proposal
Introduce `Epic` as a list option within a _Group Issue Boards_ with the expectation that the list would contain issues which are children of that epic. :

## :warning: Feature blocked
- [x] This feature is blocked by the [boards migration to Apollo](https://gitlab.com/groups/gitlab-org/-/epics/9026).
- [x] This feature is blocked by [epic migration to work items](https://gitlab.com/groups/gitlab-org/-/epics/9290)
- [ ] This feature is blocked by [issue migration to work items](https://gitlab.com/groups/gitlab-org/-/epics/9584)
epic