Create a new email sender.

Body shape: { "type": "smtp" | "ses", "label": "Gmail", "from_address": "reports@example.com", "from_name": "Analytics Reports", // optional "is_default": false, // optional "config": { // for smtp: host, port, username?, password?, tls_mode // for ses: region, access_key_id, secret_access_key, configuration_set? } }