Add CI/CD test and build pipeline

[happy-turtle]

This is an issue to track the setup of a CI/CD pipeline. Automatic package tests and build outputs would help a lot to ensure package stability and speed up the development.

The GameCI project looks very promising, but the support for packages is limited and requires too much manual setup at the moment. Some issues that currently block an easy integration are:

• Generating a unitypackage using unity-builder game-ci/unity-builder#236
• Feature: Allow unityVersion of auto for Package Testing game-ci/unity-test-runner#227
• Feature: Add Caching for Package Testing game-ci/unity-test-runner#226
• Package Testing Hangs if Package Being Tested is the Root Directory of the Repository game-ci/unity-test-runner#223