Upsert contact

curl --request PUT \
  --url https://app.unsend.dev/api/v1/contactBooks/{contactBookId}/contacts/{contactId} \
  --header 'Content-Type: application/json' \
  --data '{
  "email": "<string>",
  "firstName": "<string>",
  "lastName": "<string>",
  "properties": {},
  "subscribed": true
}'

{
  "contactId": "<string>"
}