ci: add a skeleton github actions workflow for downstream projects

[flavorjones]

This is one idea to start a CI suite for downstream tests, see the conversation at #141.

The goal here is to have a skeleton workflow that can be manually inspected by the maintainers and merged onto master. Once this file is on master, then subsequent PRs that modify the workflow file will (unless configured otherwise) run the workflow on the PR, which is the end goal of #141.

This workflow follows some practices that I've found useful:

• the concurrency section means that if a push to an existing PR or to master happens while the workflow is running, the run is canceled and new one is kicked off
• on: workflow_dispatch means that you can kick off a test run manually via Github's web UI
• the on: push section will run the workflow on any push to master
• the on: pull_request section will run the workflow just once for any change to a PR ("open" or "synchronize") -- by default GA will run the workflow twice when commits are pushed onto an existing PR

And the final jobs section is just a placeholder job that can be removed after this is merged.