New Feature: Unioning source data

[pawelngei]

Are you a current Fivetran customer? Yes, Pawel Ngei, Pattern Brands

What change(s) does this PR introduce?

• Introduce the ability to union data from multiple schemas databases, as per https://github.com/fivetran/dbt_klaviyo/pull/11 (for Klaviyo)
• Based on the Google Ads source plugin, https://github.com/fivetran/dbt_google_ads_source/pull/21

Does this PR introduce a breaking change?

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

Is this PR in response to a previously created Issue

• [ ] Yes
• [x] No

How did you test the PR changes?

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

Local Pattern Brands data

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

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

:musical_keyboard:

Notes

Needs a change in package.yml once google_ads_source is published.

I wasn't able to generate docs not knowing what profile I should use. Maybe you could add it to the readme?

I'll add the links to the changelog shortly.

[fivetran-joemarkiewicz]

Thanks again so much for putting the effort into this PR @pawelngei! There are a few minor changes (tests and formatting updates) that I am going to apply before we include this in a live version of the package. Therefore I am changing the base branch to be a working branch that I will apply these updates to.

Thanks so much!