Fix return value of snowflake get_table_names

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3. Please describe how your code solves the related issue.

According to the interface, the method get_table_names should return a list of strings. See: https://github.com/sqlalchemy/sqlalchemy/blob/6a87d619d9dfe82a8135f0ab3268bf526ac42383/lib/sqlalchemy/engine/reflection.py#L377-L379

The return type was updated last week by #556

We found this issue due to the result dict_keys no longer being JSON serializable. We already addressed the issue on our side great-expectations/cloud#601 and would like to contribute this fix back to the community :)

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