Find tasks across the application

In order to make the bundle usable, tasks should firstly be discovered (dump or register them with crontab is not in the scope of this PR). Finders will be created and they should look into different places (Command, config, etc).

- [x] Create a TaskFinderInterface
- [x] Create a CommandFinder that looks for commands implementing SchedulableInterface
- [x] Create a ConfigFinder that looks into configuration (pyrech_cron.tasks ?)
- [x] Tests
- [x] Docs

Annotations could also be proposed to easily configure Command...

Furthermore, nothing forbid to have an orm config that transforms Task model into Entity. We could then have an OrmFinder that load tasks from the database...