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

[finchxxia]

To solve issue #2150

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

[iffyio]

Hi @finchxxia!

• it doesn't seem like we have tests (unless I missed them) for the multitable insert diffs? If not can we add tests that demonstrate the changes in behavior
• Can we split this PR into two, one for the multitable insert and the other for the sample clause? I think that would make the PR easier to get to, given that they're unrelated changes