[FEAT] Validate phone number for subscription

[acthelemann]

⚠️ IMPORTANT: Please fill out this template to give us as much information as possible to consider/implement the feature.

Prerequisites

• [x] check this box if you have completed the following:
  • Reviewed the contributing guidelines and support files
  • Reviewed the README file for the repository you are working in
  • Searched for relevant instructions on our Discord server
  • Searched the issues of the repository you are working in to make sure one was not already filed

Summary

In the subscribe lambda:

• [ ] Validate the phone number is of E.164 format
• [ ] If the phone number is not valid, respond with a 400 status code

Motivation

Phone numbers being subscribed to the notification service should be of a consistent format and should be able to support international numbers.

Possible Alternatives

Only do this check on the frontend.

Additional Context

E.164 format