Implement Group entity

[ivlevdenis]

https://github.com/toggl/toggl_api_docs/blob/master/chapters/groups.md

[nifadyev]

Groups are moved into /organizations/<id>/ and /organizations/<id>/workspaces/<id>/ paths, so refactoring is required

[nifadyev]

Rough plan:

• Necessary endpoints (available on free plan):
  • GET/POST/DELETE /workspaces/{workspace_id}/project_groups
  • GET/POST/PATCH/DELETE /organizations/{organization_id}/groups
  • GET /organizations/{organization_id}/workspaces/{workspace_id}/groups
  • GET/POST/DELETE /workspaces/{workspace_id}/project_groups
• Cover new logic with tests and type annotations