Allow use of multiple column unique keys in snapshots

[gshank]

resolves #181
related to dbt-labs/dbt-core#10795

Problem

Implement use of a list for a unique_key in snapshots

Solution

Check whether the unique_key is a list and modify the SQL.

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

[mikealfare]

I added a few suggestions focusing on minimizing the amount of jinja that we have and making that jinja more readable. Let me know what you think.

[mikealfare]

Every time we remove jinja, an angel gets its wings.