Add support for Snowflake CLI alongside SnowSQL

- Add g:db_adapter_snowflake_use_cli variable to toggle between tools - Switch to long cli args to simplify command invocation (same args for both) - Switch to csv completion for the same reason (snow cli doesn't support tsv) - Keep SnowSQL as default for backward compatibility - Add tests to ensure functionality Relates to https://github.com/tpope/vim-dadbod/pull/174