Snowflake: does not support statement: Insert(Multi-table)

[finchxxia]

I think we should support updates multiple tables by inserting one or more rows with column values (from a query) into the tables. Supports both unconditional and conditional inserts.

Example:

INSERT ALL
  INTO t1
  INTO t1 (c1, c2, c3) VALUES (n2, n1, DEFAULT)
  INTO t2 (c1, c2, c3)
  INTO t2 VALUES (n3, n2, n1)
SELECT n1, n2, n3 from src;

-- If t1 and t2 need to be truncated before inserting, OVERWRITE must be specified
INSERT OVERWRITE ALL
  INTO t1
  INTO t1 (c1, c2, c3) VALUES (n2, n1, DEFAULT)
  INTO t2 (c1, c2, c3)
  INTO t2 VALUES (n3, n2, n1)
SELECT n1, n2, n3 from src;