Add compatibility for JSON output of dbt>=0.19.0.

Summary:
Intended to resolve issue #3616

The JSON schema for dbt run results (among many other dbt Artifacts) has changed in dbt 0.19.0.

dagster-dbt currently fails when parsing the output from dbt run and dbt compile. This diff sets missing fields as optional and should be compatible with dbt before and after 0.19.0

To Do

• Decide on how long dagster-dbt will support dbt <0.19.0. Please comment below with your thoughts.
• Include new metadata fields from dbt 0.19.0 in the dagster-dbt Outputs and AssetMaterializations

Test Plan: buildkite

Reviewers: sandyryza, max

Reviewed By: max

Differential Revision: https://dagster.phacility.com/D6407