union feature updates

[fivetran-joemarkiewicz]

Are you a current Fivetran customer?

Fivetran created PR

What change(s) does this PR introduce?

This PR adds the union ability to the following platforms

• Facebook
• Google

Does this PR introduce a breaking change?

• [X] Yes (please provide breaking change details below.)
• [ ] No (please provide explanation how the change is non breaking below.)

This will be a breaking change for the existing users of the dbt package.

Is this PR in response to a previously created Issue

• [ ] Yes, Issue [link issue number here]
• [X] No

How did you test the PR changes?

• [X] CircleCi
• [ ] Other (please provide additional testing details below)

Select which warehouse(s) were used to test the PR

• [X] BigQuery
• [X] Redshift
• [X] Snowflake
• [X] Postgres
• [X] Databricks
• [ ] Other (provide details below)

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We are so excited you decided to contribute to the Fivetran community dbt package! We continue to work to improve the packages and would greatly appreciate your feedback on our existing dbt packages or what you'd like to see next.