Add ability to add specify column null and default values from cli

[TheSmartnik]

Hi guys,

I use rails generators extensively and was surprised that i can't specify null and default values.

It seems like a really nice feature to have, so I've created a pr for this https://github.com/rails/rails/pull/31448.

It's been almost a month with no feedback. It'd be great if someone could take a look at it.