[FEATURE] Support non-standard character for cron expressions (e.g. L and W character)

**Is your feature request related to a problem? Please describe.**
When setting up cron scheduler with special character `L` to make the cron run on example last sunday of month it wont work on Pipedream as Pipedream doesn't support special characters that are non-standard.

**Describe the solution you'd like**
Possibility to use L and W character in cron expressions, so example this cron expression would work:
`0 6 * * 7L` (At 06:00 AM, on the last Sunday of the month).
Can validate cron here:
https://crontab.cronhub.io/

**Do you have a workaround?**
`date-fns` has been suggested here: https://pipedream.com/community/t/how-can-i-make-cron-run-on-the-last-sunday-of-the-month/1847/4
Haven't tested yet.

**Comparable features in other tools?**
https://www.easycron.com/ (Seems to work here)

**Additional context**
Can validate cron expressions here, L and W works:
https://crontab.cronhub.io/

For reference:
https://help.sumologic.com/03Send-Data/Sources/01Sources-for-Installed-Collectors/Script-Source/Cron-Examples-and-Reference