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) **Provide an emoji that best describes your current mood** 🔗 **Feedback** 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](https://www.surveymonkey.com/r/DQ7K7WW) on our existing dbt packages or what you'd like to see next.

[fivetran-joemarkiewicz]

Closing this PR as it is quite stale in comparison to the latest version of the package.