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 Facebook Ads Source plugin: https://github.com/fivetran/dbt_facebook_ads_source/pull/18 and https://github.com/fivetran/dbt_facebook_ads_creative_history/pull/16

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, Issue [link issue number here]
• [x] No

How did you test the PR changes?

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

Pattern Brands data, two datasets from Facebook Ads connector with Snowflake.

IMPORTANT: Our data didn't generate anything in the stg_facebook_ads__app_link model, you might want to test it on another dataset.

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

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

:building_construction:

Feedback

Added info about the basic_ad report to this plugin's Readme, as a lot of your potential users might not look at the source plugin (which doesn't need to be installed separately).

Needs new auto-generated docs and the packages.yml change once the _source and _creative_history plugins are published. Added a basic Changelog, you might want to build on it.

[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!