support hierarchical json format in json datasource

[xiajingchun]

Today, json datasource only supports "flat" format, e.g., each line of the json file should be like:

 {
"key1:"value1, 
"key2:"value2",
...
}

In some case, the json file could use hierarchical structure, the value of a key is a child-object or even an array of objects, e.g.

{
"key1":
       [
           {"key11":"value11", "key12":"value12"},
           {"key21":"value21", "key22":"value22"},
           ...
       ]
}

Can we add support for this?

[xiajingchun]

Regarding the array, it's better to support a mapping like below:
tag.prop1 <-> json["key1"][0]["key11"]
tag.prop2 <-> json["key1"][1]["key22"]
It's up to the user how to design the json object.