Snowflake COPY INTO target columns, select items and optional alias

[yoavcloud]

This PR adds support for the following in Snowflake's COPY INTO:

1. Specifying a list of columns in the destination table:

COPY INTO [<namespace>.]<table_name> [ ( <col_name> [ , <col_name> ... ] ) ]

2. Using regular SQL expressions in the data transformation query, typically for constant or computed values:

SELECT t1.$1:st AS st, $1:index, t2.$1, 4, '5' AS const_str FROM @schema.general_finished

3. The AS keyword is optional when specifying an alias for the stage in the data transformation query

COPY INTO tbl FROM (SELECT t1.$1:st AS st, 4, '5' AS const_str FROM @stage S)

[iffyio]

could we do something like this in order to avoid the token clone here?

if matches!(parser.peek_token_ref(), Token::Comma) {
    parser.advance()
} else {
   break
}

[yoavcloud]

Great idea! I included it in the latest commit.

[iffyio]

LGTM! Thanks @yoavcloud!
cc @alamb