[CX] extend object_concat to support an input array

Currently the function object_concat takes an arbitrary but fixed number of JSON objects as parameters and aggregates them to one JSON object that contains all key-value pairs of the input JSON objects.

To support more dynamic uses of the function, we should also support passing an array of objects as a (single) parameter. This allows us to

1. concatenate an arbitrary number of JSON objects into a single object and
2. the sequence of input objects can be computed at runtime (e.g. by a subquery).

This allows us to dynamically create new objects. E.g.

SELECT VALUE OBJECT_CONCAT([{"name1":"val1"},{"name2":"val2"}])

would yield

[

  {

    "name1": "val1",

    "name2": "val2"

  }

]

just like

SELECT VALUE OBJECT o.name: o.val FOR o IN

    [{"name":"name1","val":"val1"},{"name":"name2","val":"val2"}]

END

which uses the more general OBJECT constructor functionality that is available in the Query Service.