feat(admin): add user and group management via Admin Directory API

Implements Google Admin SDK Directory API support for Workspace user and

group management with domain-wide delegation.

New commands:

- gog admin users list --domain example.com

- gog admin users get user@example.com

- gog admin users create user@example.com --given John --family Doe

- gog admin users suspend user@example.com

- gog admin groups list --domain example.com

- gog admin groups members list group@example.com

- gog admin groups members add group@example.com user@example.com --role MEMBER

- gog admin groups members remove group@example.com user@example.com

Closes #340

Note: This is a focused implementation of core user/group management.

For full GAM feature parity (PR #179), additional work would be needed

for: alerts, licenses, org units, printers, reports, vault, SSO, etc.

Author: dl-alexandre