feat(groups): add require_sha_for_merge fields to Group

Adds RequireSHAForMerge and LockRequireSHAForMerge to the Group response struct. GitLab returns them from the group detail endpoints. They tell whether merge requests in a group require a valid commit SHA when merged, and whether that setting is enforced for descendant groups.

GitLab API docs: https://docs.gitlab.com/api/groups/

Testing

TestGetGroup now returns both fields and checks they are decoded onto Group.

Closes #2321 (closed)