Find tasks across the application

- [ ] Create a TaskFinderInterface
- [ ] Create a CommandFinder that looks for commands implementing SchedulableInterface
- [ ] Tests
- [ ] Docs

Furthermore, nothing forbid to have a config that transform Task into Entitie. We could then have a DoctrineFinder that load tasks from the database...

Annotations could also be created to easily configure Command, and an AnnotationFinder coulb be created...