Snowflake: add Snowflake multi table insert support & add support for sample in subquery

[finchxxia]

1. I found that datafusion-sqlparser-rs cannot support multi-table insert currently.

-- Unconditional multi-table insert
INSERT [ OVERWRITE ] ALL
  intoClause [ ... ]
<subquery>

-- Conditional multi-table insert
INSERT [ OVERWRITE ] { FIRST | ALL }
  { WHEN <condition> THEN intoClause [ ... ] }
  [ ... ]
  [ ELSE intoClause ]
<subquery>

2. Another case is when I using SAMPLE in subquery, it cannot work normally.

SELECT * FROM (SELECT * FROM (select report_from from my_table) SAMPLE (10000 ROWS)) AS anon_1