Fix SQL syntax for files parameter in CopyFromExternalStageToSnowflakeOperator

[bnix]

Currently, CopyFromExternalStageToSnowflakeOperator will return a SQL compilation error if a file list is specified. The optional FILES parameter for Snowflake's COPY INTO command requires the file list to be surrounded by parentheses.

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[boring-cyborg]

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[boring-cyborg]

Awesome work, congrats on your first merged pull request!