FR: Support validation policies on SMS setup stages

**Is your feature request related to a problem? Please describe.**
I would like to restrict the country code phone numbers are accepted from with a validation policy so that people cannot verify out-of-country phone numbers.

**Describe the solution you'd like**
Ability to use policies with SMS stages.

**Additional context**
Currently it's a little troublesome that one cannot simply attach an validator to the stage that asks for the phone number, but rather it would accept any phone number, which is not always wanted.