SQL: SIGNUM cannot be used in script context

[imotov]

Due to the name mismatch between

elasticsearch/x-pack/plugin/sql/src/main/java/org/elasticsearch/xpack/sql/expression/function/scalar/math/Sign.java

Lines 37 to 40 in 2efd224

	@Override
	protected String mathFunction() {
	return "signum";
	}

and

elasticsearch/x-pack/plugin/sql/src/main/java/org/elasticsearch/xpack/sql/expression/function/scalar/whitelist/InternalSqlScriptUtils.java

Lines 260 to 262 in 08da5bb

	public static Double sign(Number value) {
	return MathOperation.SIGN.apply(value);
	}

The SIGN/SIGNUM function fail when used in GROUP BY or ORDER BY.

To reproduce:

SELECT SIGNUM(salary - 50000) sig FROM test_emp GROUP BY sig 

{
  "error": {
    "root_cause": [
      {
        "type": "script_exception",
        "reason": "compile error",
        "script_stack": [
          "InternalSqlScriptUtils.signum(InternalSqlScript ...",
          "                      ^---- HERE"
        ],
        "script": "InternalSqlScriptUtils.signum(InternalSqlScriptUtils.sub(InternalSqlScriptUtils.docValue(doc,params.v0),params.v1))",
        "lang": "painless"
      }
    ],
    "type": "search_phase_execution_exception",
    "reason": "all shards failed",
    "phase": "query",
    "grouped": true,
    "failed_shards": [
      {
        "shard": 0,
        "index": "test_emp",
        "node": "uzOJf9yTSGGYtJILcwpG5Q",
        "reason": {
          "type": "script_exception",
          "reason": "compile error",
          "script_stack": [
            "InternalSqlScriptUtils.signum(InternalSqlScript ...",
            "                      ^---- HERE"
          ],
          "script": "InternalSqlScriptUtils.signum(InternalSqlScriptUtils.sub(InternalSqlScriptUtils.docValue(doc,params.v0),params.v1))",
          "lang": "painless",
          "caused_by": {
            "type": "illegal_argument_exception",
            "reason": "method [org.elasticsearch.xpack.sql.expression.function.scalar.whitelist.InternalSqlScriptUtils, signum/1] not found"
          }
        }
      }
    ]
  },
  "status": 400
}

[elasticmachine]

Pinging @elastic/es-search-aggs