Add a contact to a campaign

Adds ONE contact to a multichannel campaign. The contact is stored in a list named "<campaign name> contacts from API" (created if needed) attached to the campaign. If the campaign is FINISHED it is started again automatically (all start checks apply); if it is RUNNING the contact is scheduled immediately. A contact already present in that list (same email or LinkedIn URL) is rejected. For many contacts, use the Lists endpoints and PATCH .../recipients instead.