[CX] object-concat() should issue a warning on duplicate fields

When object-concat is applied on multiple objects with different keys, e.g.

select value object_concat({"a":"b"}, {"b":"c"});

the objects are concatenated as expected

[

  {

    "b": "c",

    "a": "b"

  }

]

But when object-concat is applied on multiple that share the same key, the value of a concatenated object is overwritten, i.e.

select value object_concat({"a":"b"}, {"a":"c"});

yields

[

  {

    "a": "c"

  }

]

which can lead to confusing behavior, if the query author is not aware of this behavior.

We should issue a warning to alert the query author.