Update to the CI/CD pipeline via github workflow to help cut turn-around time.

• added github workflow
• changed the number of pytest-xdist workers from "auto" to "8" (based on local tests and discussion with Tim, choosing auto sometimes takes longer to run than choosing a smaller number. I suspect this is partly because for small or short tests the overhead needed to setup a worker exceeds the time savings of having extra workers).
• modified numerous tests to explicitly include a project path to avoid an attempt to find the project by making an external call via the pydata-google-auth workflow (which opens an input and waits for response from the user that never comes).