Split TodoService into smaller specialized services

The following discussion from !180737 (merged) should be addressed:

• @eduardobonet started a discussion:

  Note Not for this MR, but for the future, we prefer having many small contained services, each with their own responsibility ad a single execute method, rather than having a large service. This grew into a very large class, maybe we should considering refactoring this?

---

The TodoService class has grown into a too large service that bears too many responsibilities. It should be refactored into smaller contained services (eg a TodoResolveService for resolving to-dos).