Feat(snowflake): improve transpilation of queries with UNNEST sources

[georgesittas]

Transpiling BigQuery queries to Snowflake is not always correct today. One example is:

-- BigQuery input
SELECT x FROM UNNEST([STRUCT('x' AS x)])

-- Snowflake output, produced by SQLGlot's transpiler
SELECT x FROM TABLE(FLATTEN(INPUT => [OBJECT_CONSTRUCT('x', 'x')])) AS _u(seq, key, path, index, value, this)

The transpiler's output is not valid Snowflake, because the identifier x referenced in the projection list does not exist. Instead, only the identifiers mentioned in the table alias are visible, and accessing x requires extracting it from value.

To address this issue, I implemented a rule that conditionally qualifies UNNEST sources and the columns they produce, if we can statically determine them.

After this rule is applied, a followup rule, already in effect today, takes care of converting the qualified columns into the proper bracket expressions used to extract values out of VARIANT objects, which leads to a correct transpilation.