Get a billing group

curl -X GET \
 /billing-groups/{id} \
 --header "Authorization: Bearer <token>"

{
  "id": "01F9ZQZJZJQZJZJZJZJZJZJZJZ",
  "name": "Billing of payments at the end of the month",
  "type": "<any>",
  "customDay": 31,
  "customMonth": 12,
  "subscriptions": [
    "https://example.com/"
  ]
}