Phone verification: add a searchable dropdown for country selector

What does this MR do and why?

• Resolves https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/85
• Improves the UI by replacing the dropdown to select the country with a searchable listbox.

Screenshots or screen recordings

Before	After
country-selector-before	country-selector-after

How to set up and validate locally

1. Turn on feature flag

Feature.enable(:arkose_labs_signup_challenge)
Feature.enable(:identity_verification_phone_number)
Feature.enable(:identity_verification)

2. Update application setting to turn on verification

ApplicationSetting.first.update(require_admin_approval_after_user_signup: false, email_confirmation_setting: "hard")

3. Get credentials from 1Password to connect to our external services

ApplicationSetting.first.update(arkose_labs_public_api_key: XX, arkose_labs_private_api_key: XX, telesign_customer_xid: XX, telesign_api_key: XX)

4. Sign-up as a new user /users/sign_up. You should see a page with content Help us keep GitLab secure.
5. Turn on phone-verification

UserCustomAttribute.where(user: User.last, key: 'arkose_risk_band').update(value: 'MEDIUM')

6. Refresh the page and try entering in your phone number to receive a verification code.

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.