use $jsonGroupArray to return an array of primitive values as JSON

[kbarbounakis]

This PR updates $jsonGroupArray dialect to return a JSON array or values e.g. [ 'orange', 'apple', 'lemon' ] when a QueryExpression is passed.

e.g.

const query = new QueryExpression().select(
                'id',
                'familyName',
                'givenName',
                'jobTitle',
                'email',
                new QueryField({
                    products: {
                        $jsonGroupArray: [
                            new QueryExpression().select(
                                new QueryField('name').from(Products),
                            ).from(Products).join(Orders).with(
                                new QueryExpression().where(
                                    new QueryField('orderedItem').from(Orders)
                                ).equal(
                                    new QueryField('id').from(Products)
                                )
                            ).where(
                                new QueryField('customer').from(Orders)
                            ).equal(
                                new QueryField('id').from(People)
                            )
                        ]
                    }
                })
            ).from(People).where('email').equal('eric.thomas@example.com');

which returns an array of purchased product names

{
  "id": 401,
  "familyName": "Thomas",
  "givenName": "Eric",
  "jobTitle": "Naval Architecture",
  "email": "eric.thomas@example.com",
  "products": [
    "Kobo Aura",
    "SanDisk Ultra Plus (256GB)",
    "Lenovo ThinkPad Tablet 2",
    "Dell XPS 27"
  ]
}

The generated SQL statement uses json_agg to execute a single-valued query

SELECT "PersonData"."id", "PersonData"."familyName", 
"PersonData"."givenName", 
"PersonData"."jobTitle", "PersonData"."email", (
  SELECT json_agg("ProductData"."name") AS "value" FROM "ProductData"
     INNER JOIN "OrderData" ON ("OrderData"."orderedItem"="ProductData"."id") 
     WHERE ("OrderData"."customer"="PersonData"."id")
) AS "products" 
FROM "PersonData" 
WHERE ("email"='eric.thomas@example.com')