Add cron-ext utility

[pnadolny13]

https://github.com/meltano/cron-ext

[sbalnojan]

@WillDaSilva how do you feel about that? Is the extension fine to use from your perspective? I'm asking because I was thinking about using it in an example or two. Cron is much simpler to explain to people than airflow especially in getting started guides or tutorials, so I would love to have that option available.

[WillDaSilva]

how do you feel about that?

@sbalnojan I'd love to see cron-ext get more use!

Is the extension fine to use from your perspective?

Yes, though it hasn't been widely used, so there may be some yet undiscovered issues with it. Please report any you find, and I'll try to fix them soon after.