[Feature] Consider the usage of json functions for getting related objects

[kbarbounakis]

Description
@themost/data module uses @themost/query#QueryExpression class for creating complex queries, getting related objects etc.
It seems very useful to optimise fetch operations and use JSON functions that are available in many database engines like MySQL, MSSQL, Oracle, Sqlite etc. Such an operation will try to get related objects as JSON objects in a single query statement prepared by the @themost/query which already supports JSON queries e.g. get order customer and ordered item as a JSON object

SELECT OrderData.id,
       json_object(
               'id', customer.id,'familyName', customer.familyName,'givenName', customer.givenName
                ) as customer,
      json_object(
               'id', orderedItem.id,'name', orderedItem.name,
               'category', orderedItem.category, 'price', orderedItem.price
                ) as orderedItem
FROM OrderData
INNER JOIN (
    SELECT id, familyName, givenName FROM PersonData
) customer ON OrderData.customer = customer.id

INNER JOIN (
    SELECT id, name, category, price FROM ProductData
) orderedItem ON OrderData.orderedItem = orderedItem.id
limit 25

The resultset of the previous query is something like the following table:

id	customer	orderedItem
1	{"id":391,"familyName":"Nicholls","givenName":"Meghan"}	{"id":35,"name":"Google Nexus 7 (2013)","category":"Tablets and E-Readers","price":401.81}
2	{"id":399,"familyName":"Hawkins","givenName":"Jesse"}	{"id":55,"name":"AMD Radeon R9 290","category":"PC Components","price":452.35}

where customer and orderedItem are serialized JSON objects.

[kbarbounakis]

A similar operation should be supported also for getting an array of related objects:

SELECT PersonData.*,
       (SELECT json_group_array(
                json_object(
               'id', OrderData.id,
               'name', OrderData.orderDate,
               'orderedItem', OrderData.orderedItem
                )
               ) FROM OrderData
                    WHERE OrderData.customer = PersonData.id) orders
       FROM PersonData limit 25

where every person has an array of orders e.g.

id	givenName	familyName	orders
349	Aaron	Matthews	[{"id":44,"name":"2019-02-24 19:29:02.000+02:00","orderedItem":86},{"id":63,"name":"2019-03-12 00:00:18.000+02:00","orderedItem":55}]
351	Gavin	Mcdonald	[{"id":24,"name":"2019-01-27 12:27:44.000+02:00","orderedItem":47},{"id":28,"name":"2019-02-24 10:50:25.000+02:00","orderedItem":8},{"id":97,"name":"2019-02-19 10:34:42.000+02:00","orderedItem":86},{"id":113,"name":"2019-02-15 23:48:13.000+02:00","orderedItem":54}]