Add groupMembers to api.js

What does this MR do?

This MR adds a method to api.js for the group/members endpoint.

Why is this needed now?

• For #1979 (moved), we need to fetch the group members in the create/edit MR so that we can expand the full set of approvers for an approval rule.
• See how it is used.

What are the relevant issue numbers?

• gitlab-ee#1979

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer