Allow to list and update epic labels

Similar as with REST API, it would be nice to list/update epic labels when getting/updating an epic.

The following discussion from !18440 (merged) should be addressed:

• @jprovaznik started a discussion:

  I omitted labels argument here, although in REST API we allow updating also labels (https://gitlab.com/gitlab-org/gitlab/blob/master/ee/lib/api/epics.rb#L95). The reason is that at the moment we don't support showing epic labels through graphql yet, so it makes more sense to address this in another issue "Allow to list and update epic labels".

/cc @johnhope