Is @foreignKey available for 2 or more columns in 1 view?
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### Summary

<!-- Please detail the issue you'd like to discuss. -->

In my understanding, `@foreingKey` smart tags can be used for only 1 column in 1 view.

```sql
CREATE TABLE product (
  product_id BIGSERIAL,
  name TEXT
);

CREATE TABLE customer (
  customer_id BIGSERIAL,
  name TEXT,
  email TEXT
);

CREATE TABLE order (
  order_id BIGSERIAL,
  product_id BIGINT NOT NULL REFERENCES product (product_id) ON UPDATE CASCADE ON DELETE RESTRICT,
  customer_id BIGINT NOT NULL REFERENCES customer (customer_id) ON UPDATE CASCADE ON DELETE RESTRICT,
  amount INT
);

CREATE VIEW some_view AS (
  SELECT product.product_id
       , customer.customer_id
       , order.order_id
  FROM order
    INNER JOIN product ON order.product_id = product.product_id
    INNER JOIN customer ON order.customer_id = customer.customer_id
);
```

In this definition above in Postgres, I can define a `@foreginKey` smart tags like this by library usage.

```typescript
import { makeJSONPgSmartTagsPlugin } from 'graphile-utils';
import { postgraphile } from 'postgraphile';

const graphqlServer = postgraphile('DB_URL', 'public', {
  // ... some other options
  appendPlugins: [
    makeJSONPgSmartTagsPlugin({
      version: 1,
      config: {
        class: {
          some_view: {
            tags: {
              foreignKey: '(order_id) references order (order_id)'
            }
          }
        }
      }
    }),
  ]
});
```

```graphql
query {
  someViews {
    nodes {
      productId
      customerId
      orderId
      order {  # <- thanks to foreignKey
        amount
      }
    }
  }
}
```

I want to add more `@foreignKey` for other tables like `product` and `customer`.

```graphql
query {
  someViews {
    nodes {
      product {  # <- foreginKey #1
        productId
        name
      }
      customer {  # <- foreginKey #2
        customerId
        name
        email
      }
      orderId
      order {  # <- foreginKey #3
        amount
      }
    }
  }
}
```

I tried these, and faced warning.

```
foreignKey: '(product_id) references product (product_id), (customer_id) references customer (customer_id), (order_id) references order (order_id)'
```

```
foreignKey: [
  '(product_id) references product (product_id)',
  '(customer_id) references customer (customer_id)',
  '(order_id) references order (order_id)'
]
```

How can I deal with it?

### Additional context

<!-- Please provide additional context such as environment versions (including
      operating system, Node, PostgreSQL, Redis, etc. as relevant), software
      versions (from `yarn.lock`) or similar, plugins in use, database schema
      if relevant, bundler (if any). -->

* NodeJS: 14
* Postgrahile: `v4.12.0-alpha.0`