scheduler: Ability to create a DAG where it is repeatable given start date, end date and CRON

For example, if have I have

```
cron: 5 * * * *
start_date: 2024-10-10
end_date: 2024-12-10
```

This DAG will generate tasks that run every 5 minutes, and generate a variable (like $CRON_DATE) that can be passed to the underlying shell commands.

This is useful when you have historical data that is divided by hours, and you want to do a bunch of processing on them.

Start by locating the scheduler path that parses CRON expressions and DAG dates, then trace how scheduled values reach underlying shell commands. Done means a DAG can repeat between the supplied start and end dates, runs at the requested CRON times, and exposes a date variable to those commands.

Written by the indexing model from the issue text.