What:

- Continuation of refactoring discussed and started in
  https://gitlab.com/gitlab-org/gitlab/-/merge_requests/30979#note_337747596
- Refactors Issuable-related service classes to subclass
  `BaseContainerService` instead of `BaseService`.

Why:

- This provides a greater degree of type safety in order to
  more easily and safely introduce a new named argument
  `spam_params` to support
  https://gitlab.com/gitlab-org/gitlab/-/merge_requests/58603
- The additional type safety will also make future refactors
  easier and less risky

How:

- Note that this is an incremental commit.
  In a subsequent commit, the
  first argument will be changed to `group` in the
  `Epics::BaseService` hierarchy.