Add support for UDFs: CREATE FUNCTION

It would be great to have support for `CREATE FUNCTION` statement to define UDFs.

I think it makes sense to develop the more minimal variant of BigQuery first: support for `LANGUAGE` (e.g. javascript) and `AS` (sql) or even sql-only to start with.

https://cloud.google.com/bigquery/docs/reference/standard-sql/user-defined-functions

https://www.postgresql.org/docs/9.1/sql-createfunction.html

首先将所请求的最小 CREATE FUNCTION 语法与链接的 BigQuery 和 PostgreSQL 参考进行比较，重点关注 LANGUAGE 和 AS。完成的标准是 parser 接受约定的纯 SQL 形式或 LANGUAGE/AS 形式，并一致地拒绝不支持的形式；issue 未指定要运行的文件或测试。

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