How to create a new dimension and fill the column with default value during ingestion

When creating a new Druid DataSource, admins may want to add new dimensions that's not in raw data, and fill the columns with default values. This can be achieved by using `transformSpec` during data ingestion. 

Here is a example of adding two dummy columns `dummyCol1` and `dummyCol2` to `Wikipedia` dataSource in ingestion spec :

{
  "type" : "index",
  "spec" : {
    "dataSchema" : {
      "dataSource" : "wikipedia",
      "parser" : {
        "type" : "string",
        "parseSpec" : {
          "format" : "json",
          "timestampSpec" : {
            "column" : "timestamp",
            "format" : "iso"
          },
          "dimensionsSpec" : {
            "dimensions" : [ "diffUrl", "isRobot", {
              "name" : "added",
              "type" : "long"
            }, .....
              "name" : "metroCode",
              "type" : "long"
            }, "dummyCol1", "dummyCol2" ]
          }
        }
      },
      "metricsSpec" : [ ],
      "granularitySpec" : {
...
        },
        "rollup" : false,
        "intervals" : null
      },
      "transformSpec" : {
        "filter" : null,
        "transforms" : [ {
          "type" : "expression",
          "name" : "dummyCol1",
          "expression" : "nvl(\"dummyCol1\", 'HAPPY')"
        }, {
          "type" : "expression",
          "name" : "dummyCol2",
          "expression" : "nvl(\"dummyCol2\", 'JOY')"
        } ]
      }
    },
    "ioConfig" : {
      "type" : "index",
      "firehose" : {
        "type" : "http",
        "uris" : [ "https://SERVER/data/wikipedia.json.gz" ],
...
      },
      "appendToExisting" : false
    },
    "tuningConfig" : {
...
    }
  },
  "dataSource" : "wikipedia"
}

After ingestion is done, we can see the two new dimensions are created :