Add postgres support

[dpguthrie]

Overview

• Adds relevant macros to enable support for Postgres
• Adds a postgres service to the github action used for integration testing
• Aliased the subquery used in the singular test as it's necessary within Postgres (should work across all other supported DWs)

Update type - breaking / non-breaking

• Minor bug fix
• Documentation improvements
• Quality of Life improvements
• New features (non-breaking change)
• New features (breaking change)
• Other (non-breaking change)
• Other (breaking change)

What does this solve?

Support for a new platform, Postgres

Outstanding questions

What databases have you tested with?

• Snowflake
• Google BigQuery
• Databricks
• Spark
• Postgres
• N/A

[dpguthrie]

Passing postgres integration tests - https://github.com/dpguthrie/dbt_artifacts/actions/runs/5538393942

[glsdown]

Hi @dpguthrie . Thanks for taking the time to add this functionality in.

One of the team will spend some time reviewing it and get back to you.

[drewblinn]

Would love to see this in the next release!

[glsdown]

Thank you for your work on this. Sorry it's taken a while to review - there have been a number of changes to improve the backend, so I wanted to make sure those had been made first.