Support list of strings for incremental unique_key

[ghost]

Description

In dbt 1.1 support was added for unique_key to be a list of strings as well as just a string. The dry runner fails to validate the manifest if a project uses this feature. This PR changes the pydantic model to allow list of strings for unique_key. It also verifies that the snapshot model does not have a list of unique_key in case dbt adds this same feature to snapshot models and we need to update the node runner to handle this.

Fixes #18

Checklist:

• I have run make verify and fixed any linting or test errors
• I have added appropriate unit tests or if applicable an integration test
• OPTIONAL: I have run make integration against a Big Query instance

[georgim0]

LG