Migrate CircleCI to GitHub Actions

[mwallace582]

resolves #120

Description

Migrate Circle CI lint, unit, and integration tests to GitHub Actions

dbt-snowflake and dbt-bigquery were used as inspiration for these changes, and I've updated this repo in many places to more closely follow those examples.

Checklist

• [x] I have run this code in development and it appears to resolve the stated issue
• [x] This PR includes tests, or tests are not required/relevant for this PR
• [x] I have updated the CHANGELOG.md with information about my change

[mwallace582]

Integration tests won't run on this PR because the pull_request_target workflow requires that the test infrastructure has merged to the destination before tests can run.

I did verify that these tests run and pass in an earlier commit where I temporarily specified the pull_request workflow instead.