Custom query expression

[kbarbounakis]

This MR adds a new feature for defining custom query expressions for readonly fields. This operation allows parsing custom query expressions and include them in data model's view adapter.
e.g. A business process needs customer's email address customer.email to be an attribute of an Order even if it would be retrieved by expanding customer.

Define a readonly attribute orderEmail

{
    "name": "orderEmail",
    "readonly": true,
    "type": "Text",
    "nullable": true,
    "query": [
        {
            "$lookup": {
                "from": "Person",
                "foreignField": "id",
                "localField": "customer",
                "as": "customer"
            }
        },
        {
            "$project": {
                "orderEmail": "$customer.email"
            }
        }
    ]
}

where orderEmail has a custom query with two stages:

1. Join Person collection based on customer attribute of an order.

{
    "$lookup": {
        "from": "Person",
        "foreignField": "id",
        "localField": "customer",
        "as": "customer"
    }
}

3. Select customer.email as part of an order

{
    "$project": {
         "orderEmail": "$customer.email"
     }
}

The result will be a new extra field based on an SQL statement equivalent to the following one:

CREATE VIEW OrderData AS SELECT ..., OrderBase.customer , customer.email AS orderEmail 
     LEFT JOIN PersonData AS customer ON OrderBase.customer = customer.id