Phone Verification: Backend TeleSign services

What does this MR do and why?

Part of the bigger epic https://gitlab.com/groups/gitlab-org/modelops/anti-abuse/-/epics/3 to ask users to verify their phone number if they are considered medium/high risk. We are going to use an external service called TeleSign to verify users phone numbers and store a risk score. This MR:

• Adds the TeleSign's customer_id & api_key to the application_settings table.
• Creates services to send the verification code to the user's phone number, verify the code, and get risk score for the phone number, all using TeleSign.

Screenshots or screen recordings

How to set up and validate locally

1. Configure Telesign. The customer_id and api_key are in 1Password.

::Gitlab::CurrentSettings.update(telesign_customer_xid: CUSTOMER_ID, telesign_api_key: API_KEY)

2. To send a verification code, enter your international phone number -> country code + phone number. eg. 61450331311 for Australia

result = PhoneVerification::TelesignClient::SendVerificationCodeService.new(phone_number: INTERNATIONAL_PHONE_NUMBER, user: User.find(1)).execute
telesign_reference_xid = result[:telesign_reference_xid]

3. To verify the code, use the telesign_reference_xid from the previous response

PhoneVerification::TelesignClient::VerifyCodeService.new(telesign_reference_id: telesign_reference_xid, verification_code: SMS_CODE, user: User.find(1)).execute

4. To get a risk score for a phone number. If you enter a blocked/invalid/unsual phone number such as 911, it should result in a 400.

PhoneVerification::TelesignClient::RiskScoreService.new(phone_number: INTERNATIONAL_PHONE_NUMBER, user: User.find(1)).execute

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/78