Support gitlab epics

[fredrikatmainspringenergy]

Describe the feature or problem you'd like to solve
It would be great if glab could support associating issues with gitlab epics, and even greater if it could also manage (create/update/delete/...) epics.

Propose a Solution

glab issue update [-e|--epic] <epic number>  # link issue to epic
glab issue update [-e|--epic] none           # unlink issue
glab epic [close|create|delete|list|...]     # life of an epic

Additional context

[clemsbot]

This issue has been automatically marked as stale because it has not had recent activity. We haven't had the time to address it yet, but we want to keep it open. This message is just a reminder for us to help triage issues.