DATETIME is not supported in Cast()

[davidcui1225]

Extension of #225

Queries that cast a field to type DATETIME return errors.

Example query:

SELECT CAST(timestamp AS DATETIME) FROM kibana_sample_data_flights LIMIT 10

Returns:

{
  "error": {
    "root_cause": [
      {
        "type": "script_exception",
        "reason": "runtime error",
        "script_stack": [
          "java.base/jdk.internal.math.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:2054)",
          "java.base/jdk.internal.math.FloatingDecimal.parseDouble(FloatingDecimal.java:110)",
          "java.base/java.lang.Double.parseDouble(Double.java:549)",
          "cast_field1 = new Date(Double.parseDouble(doc['timestamp'].value.toString()).longValue()); ",
          "                                                                ^---- HERE"
        ],
        "script": "def cast_field1 = new Date(Double.parseDouble(doc['timestamp'].value.toString()).longValue()); return cast_field1",
        "lang": "painless"
      }
    ],
    "type": "search_phase_execution_exception",
    "reason": "all shards failed",
    "phase": "query",
    "grouped": true,
    "failed_shards": [
      {
        "shard": 0,
        "index": "kibana_sample_data_flights",
        "node": "3ZdEGtaiRt2uUzeNPRmNZQ",
        "reason": {
          "type": "script_exception",
          "reason": "runtime error",
          "script_stack": [
            "java.base/jdk.internal.math.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:2054)",
            "java.base/jdk.internal.math.FloatingDecimal.parseDouble(FloatingDecimal.java:110)",
            "java.base/java.lang.Double.parseDouble(Double.java:549)",
            "cast_field1 = new Date(Double.parseDouble(doc['timestamp'].value.toString()).longValue()); ",
            "                                                                ^---- HERE"
          ],
          "script": "def cast_field1 = new Date(Double.parseDouble(doc['timestamp'].value.toString()).longValue()); return cast_field1",
          "lang": "painless",
          "caused_by": {
            "type": "number_format_exception",
            "reason": "For input string: \"2019-07-22T00:00:00.000Z\""
          }
        }
      }
    ]
  },
  "status": 400
}