Add enum for matching progress events to build steps

[TadCordle]

Part of #1054. This PR adds a BuildStepType enum, which is passed to progress events so that jib-core users may know which progress events correspond to which steps in the build.

I'm still not totally familiar with the progress system, so let me know if this seems like an ok direction to take.

Possible followups:

• Add documentation for what each of the BuildStepTypes mean
• Add an event for counting the number of layers involved in the push/pull steps, or modify/add progress events that make it possible to identify which layer a progress event was sent from

[TadCordle]

Going to add some tests, so no need to review thoroughly yet, but still open to high-level feedback.