fix(snowflake): COPY Subquery postfix

[VaggelisD]

Fixes #3434

This PR addresses the following issues:

• COPY would only allow files in the FROM clause, extend it to allow subqueries as well

• When parsing staged files such as @foo we'd also consume the following parentheses coming from (optional) staged file options such as SELECT * FROM @foo (FILE_FORMAT => ...). However, if there are no options specified we would end up consuming any following R_PAREN as well e.g. SELECT * FROM (SELECT * FROM @foo)

• For transforming semi-structured data Snowflake specifies the subquery columns as [<alias>.]$<file_col_num>[.<element>]. However, the tokenizer always appends the TokenType.DOT to following a number e.g. $1.elem -> $1.e lem. To solve this issue, the tokenizer will only parse an integer (and not a real) if the number is prefixed by a token of type TokenType.PARAMETER

• The base parser parses exp.Parameter as '[' <this> ':' <expression> ']' but only generates this. Snowflake parsers $<num> as exp.Parameter which can optionally be followed by a path & element name such as $1:a.c. For this reason, exp.Parameter now generates <this> [':' <expression>]

Docs

• Snowflake COPY
• Snowflake COPY Transform Subquery specs
• Snowflake Querying semi-structured data

[georgesittas]

LGTM, thanks for addressing the comments 👍