Parallel steps in GitHub Actions [GA]

### Value Prop

GitHub Actions will address the long-standing community ask to support running multiple steps in parallel in GitHub Actions

### Expected Outcome

There will be two primary feature components:
1. The ability to designate some steps as `background` steps, which will allow them to run without blocking the start of subsequent steps. We will also add additional `wait` steps where users can synchronize sequential execution and `cancel` steps that allow the graceful termination of long running `background` steps
2. A `parallel` block that will enable customers to designate a set of steps that will run in parallel. This block will be syntactic sugar that uses `background` steps and a `wait` command under the hood.