Add a contact to an email campaign

Adds one contact to the campaign (id). The contact is appended to a dedicated list named '<campaign name> from Zapier/Integromat/n8n Hooks'. Any extra key in contact beyond email/firstName/lastName/phoneNumber is stored as a custom variable. Side effects: if the campaign is FINISHED it is restarted (status RUNNING); if RUNNING the contact is scheduled immediately. Fails if the email is already in the campaign. New integrations should use the multichannel /advanced campaign endpoints instead.