Extra metadata to handle `@ref` relations for non-nullable relations plugin
A plugin that makes not-null SQL relations non-nullable might look something like this:

```ts
export const NonNullRelationsPlugin: GraphileConfig.Plugin = {
  name: 'NonNullRelationsPlugin',
  version: '1.0.0',
  schema: {
    hooks: {
      GraphQLObjectType_fields_field(field, build, context) {
        const {
          graphql: { GraphQLNonNull, getNullableType },
          input: { pgRegistry },
        } = build;
        const { isPgSingleRelationField, pgRelationDetails } = context.scope;
        if (isPgSingleRelationField && pgRelationDetails) {
          const { codec, relationName } = pgRelationDetails;
          const relation = pgRegistry.pgRelations?.[codec.name]?.[relationName];
          const everyColumnIsNonNull = relation?.localAttributes.every(
            (attrName: string) => codec.attributes[attrName]?.notNull,
          );
          if (!relation?.isReferencee && everyColumnIsNonNull) {
            field.type = new GraphQLNonNull(getNullableType(field.type));
          }
        }
        return field;
      },
    },
  },
};
```

This works great; however, a question stands on how to adapt this plugin to support the `@ref` smart tag and make relations using not-null columns also non-nullable for an SQL like:

```sql
create table people (
  id serial primary key,
  name text not null
);

create table posts (
  id serial primary key,
  author_id text not null -- 👈 the column is not null
);

comment on table posts is $$
@ref author via:(author_id)->people(id) singular
$$;
```