Validate users' phone numbers appropriately in twilio-keycloak-provider

[bickelj]

When a user logs in with the twilio-keycloak-provider configured, a user attribute of mobile_number is required and used to send an SMS. At SMS send time, there is already appropriate validation because Twilio will return an error that successfully propagates out to the keycloak UI such that the user can see it:

This issue is to validate phone numbers in other places, such as on entry of the number or at other appropriate times in user setup or validation. @jasonaowen said:

I've used Google's libphonenumber before, and it appears to have Java bindings.

Another option is Twilio's separate API for phone number validation.