Create backend phone validation feature

[terrazoon]

https://docs.aws.amazon.com/pinpoint/latest/developerguide/validate-phone-numbers.html

We want paid users to be able to check if phone numbers are valid mobile numbers before sending. The UI portion of this is TBD, but create the backend ability to validate the phone numbers right now.

[terrazoon]

Create the ability to validate a phone number. When finished, make a follow-up ticket to do what the TBD UI needs to do to make this available to paid users.