Type information is lost in JDBC format

[dai-chen]

In JDBC formatter, we need to return a type for each field in SELECT which is required by JDBC driver. However, this type is hardcoding for now. For example, SUM always return double even if the field given is an integer.

GET kibana_sample_data_flights/_mapping
{
  "kibana_sample_data_flights" : {
    "mappings" : {
      "properties" : {
        ...
        "FlightDelayMin" : {
          "type" : "integer"
        },
       ...
      }
    }
  }
}

POST _opendistro/_sql?format=jdbc
{
  "query": "SELECT SUM(FlightDelayMin) FROM kibana_sample_data_flights"
}
{
  "schema": [{
    "name": "SUM(FlightDelayMin)",
    "type": "double"
  }],
  "total": 1,
  "datarows": [["618150.0"]],
  "size": 1,
  "status": 200
}