Feat: 2-Step verification for adding contacts via API (GDPR)

[surjithctly]

To comply with GDPR, some customers needs a 2-step verification before adding to a contact list.

So if enabled, it should send a verification link for them to click to add to the list (like mailchimp)

[KMKoushik]

Yes, this is in roadmap, but if someone has a proposal, i can review