Dbt: Add `partitions` to bq AdapterSpecificConfigs

[jtcohen6]

Describe the feature

• Allow the user to define partitions in dbt_project.yml. This is a necessary config if the user wants to run incremental models with the insert_overwrite strategy for a static (user-defined) set of partitions
• Adding/overriding partitions in dbt_project.yml is the only way to adjust behavior for models imported from a package (e.g. Snowplow)

Additional context

This is functionality we added in #2198, but forgot to add partitions to AdapterSpecificConfigs in dbt-bigquery

Who will this benefit?

BigQuery users

[drewbanin]

Cool idea @jtcohen6 - let's do it!

[beckjake]

We should do this for 0.16.1, right?

[drewbanin]

closed by #2280