Adding support for v2 subscription endpoint

[mylescc]

What:

• Adding support for the v2 subscription group endpoint

Why:

• To ensure the gem is up to date.
• Docs: https://www.braze.com/docs/api/endpoints/subscription_groups/post_update_user_subscription_group_status_v2/

[mylescc]

Copied the fixes for the export_users_spec.rb from #52