Allows snapshots to have a list as a unique key

[AGPapa]

resolves #181

Problem

Currently the unique_key must be a string. It can be a column name or an expression. This PR changes it so it can be a list of column names or expressions.

Solution

This is modeled off of this PR: dbt-labs/dbt-core#4618

It checks if the unique_key is a list and modifies the code based on that. I am open to other approaches if we want something different (maybe turning a string unique_key into an array one element long?).

I couldn't find tests for the existing snapshot macros, so I'm also open to feedback on the best way to add tests for this feature. I have tested this by copying the macros to one of my dbt projects and running it with a modified version of dbt-core that allows a list for the unique_key.

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
• [s] This PR includes tests, or tests are not required/relevant for this PR
• [s] 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

[gshank]

AGPapa: We need a suite of tests for this, including for the supported adapters. At this point that's kind of a complicated process, so we're going to take your supplied patch and manage the rest of it internally. We've put this on the schedule to be estimated and scheduled, and will keep you informed.

[AGPapa]

Thank you @gshank - let me know