Support new DDL `aggregation function` syntax

[takegue]

https://cloud.google.com/bigquery/docs/reference/standard-sql/data-definition-language#sql-create-udaf-function

sql

CREATE
  [ OR REPLACE ]
  [ { TEMPORARY | TEMP } ]
  AGGREGATE FUNCTION
  [ IF NOT EXISTS ]
  function_path ( [ function_parameter[, ...] ] )
  [ RETURNS data_type ]
  AS ( sql_function_body )
  [ OPTIONS ( [function_option_list](https://cloud.google.com/bigquery/docs/reference/standard-sql/data-definition-language#function_option_list) ) ]

function_path:
  [[project_name.]dataset_name.]function_name

function_parameter:
  parameter_name
  data_type
  [ NOT AGGREGATE ]

example

CREATE AGGREGATE FUNCTION myProject.myDataset.ScaledSum(
  dividend FLOAT64,
  divisor FLOAT64 NOT AGGREGATE)
RETURNS FLOAT64
AS (
  SUM(dividend) / divisor
);

-- Call the SQL UDAF.
SELECT ScaledSum(col1, 2) AS scaled_sum
FROM (
  SELECT 1 AS col1 UNION ALL
  SELECT 3 AS col1 UNION ALL
  SELECT 5 AS col1
);

/*------------*
 | scaled_sum |
 +------------+
 | 4.5        |
 *------------*/

javascript

CREATE
  [ OR REPLACE ]
  [ { TEMPORARY | TEMP } ]
  AGGREGATE FUNCTION
  [ IF NOT EXISTS ]
  function_path([ function_parameter[, ...] ])
  RETURNS return_data_type
  LANGUAGE js
  [ OPTIONS ( [function_option_list](https://cloud.google.com/bigquery/docs/reference/standard-sql/data-definition-language#javascript-udaf-function-option-list) ) ]
  AS function_body

function_path:
  [[project_name.]dataset_name.]function_name

function_parameter:
  parameter_name parameter_data_type [ NOT AGGREGATE ]

CREATE TEMP AGGREGATE FUNCTION SumPositive(x FLOAT64)
RETURNS FLOAT64
LANGUAGE js
AS r'''
  export function initialState() {
    return {sum: 0}
  }
  export function aggregate(state, x) {
    if (x > 0) {
      state.sum += x;
    }
  }
  export function merge(state, partialState) {
    state.sum += partialState.sum;
  }
  export function finalize(state) {
    return state.sum;
  }
''';

-- Call the JavaScript UDAF.
WITH numbers AS (
  SELECT * FROM UNNEST([1.0, -1.0, 3.0, -3.0, 5.0, -5.0]) AS x)
SELECT SumPositive(x) AS sum
FROM numbers;

/*-----*
 | sum |
 +-----+
 | 9.0 |
 *-----*/