Snowflake: add order_by_keys config

[gwerbin-tive]

This supports custom ordering, which might help with downstream query optimization in some cases.

For example, you could cluster on columns (a,b) and order by (a,c), or (a,b,c).

resolves #735 (ADAP-548)

Problem

Setting cluster_by_keys also forces an order by using the same columns. This is not always desirable.

Solution

Add a new config to control clustering and ordering separately. The user can explicitly pass order_by_keys=none to disable the default behavior, or pass a column name (or list of column names) to order by some other set of columns.

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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[github-actions]

Thank you for your pull request! We could not find a changelog entry for this change in the dbt-snowflake package. For details on how to document a change, see the Contributing Guide.

[dbeatty10]

@gwerbin-tive Could you sign our Contributor License Agreement (if you haven't already)? Background here.

[gwerbin-tive]

BTW I'd appreciate advice on getting started with testing in the dbt-adapters repo. We don't write many custom macros at Tive and we don't have a formal testing process for the few that we do have, which are usually less complicated than this.