Adds refresh mode to snowflake-dbt

[matsavage]

resolves #
docs dbt-labs/docs.getdbt.com/#1086

Problem

I feel this is the bulk of the (non-test) changes required by the feature I proposed, all current unit tests pass, but I haven't added any explicitly for the refresh mode

Solution

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX

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