Nuevo endpoint para obtener subscriptions de un customer

Se agrega un nuevo endpoint para obtener las subscriptions de un customer: GET /subscriptions/customer/{customer_id}

A continuación se muestra un ejemplo de respuesta de la API:

[
    {
        "product_id": "test-product-1",
        "customer": "cus_hlbu_sXY0JUsFY9FfgkfAloN3Pl4SkCw",
        "pac_mandate_id": null,
        "is_recurring": false,
        "amount": null,
        "due_day": null,
        "receipt_product_code": null,
        "price_change": null,
        "id_price_change": null,
        "metadata": {},
        "id": "sub_hlbu_sXY0JUsFY9FfgkfAloN3Pl4SkCw"
    },
    {
        "product_id": "test-product-2",
        "customer": "cus_hlbu_sXY0JUsFY9FfgkfAloN3Pl4SkCw",
        "pac_mandate_id": null,
        "is_recurring": false,
        "amount": null,
        "due_day": null,
        "receipt_product_code": null,
        "price_change": null,
        "id_price_change": null,
        "metadata": {},
        "id": "sub_rhM4LL__sQao2Kz6xd2eIHcZxvpMTox1"
    }
]