feat(todos): add Group field to Todo

GitLab returns a group object on group-scoped to-do items (e.g. an Epic mention), the same way it returns project for project-scoped ones, but Todo had no field to decode it into.

Adds Group *ProjectNamespace, matching the REST NamespaceBasic entity field for field (id, name, path, kind, full_path, parent_id, avatar_url, web_url) the way Project.Namespace already does, and the way the Rails side serializes it (API::Entities::NamespaceBasic in basic_project_details.rb).

Flagged by @viktomas in gitlab-org/cli!3255: glab's todo list had grown a local wrapper type to carry this field because the SDK dropped it, which we'd rather fix here than work around downstream.

No new types, one field, a group-scoped fixture row in testdata/list_todos.json, and the matching expectation in TestListTodos. gofumpt clean; package tests pass (CGO_ENABLED=0 go test .).