Set up jib-build-plan sub-project

[chanseokoh]

Set up a new subproject jib-container-build-plan. It will host Java API (classes) for the Container Build Plan spec.

Brief heads-up for what it will host

Its classes will include (basically) AbsoluteUnixPath, RelativeUnixPath, Port, FilePermissions, LayerEntry, LayerConfiguration, ImageFormat, etc. They will be duplicated (after renaming if desired) from jib-core into a different package com.google.cloud.tools.jib.buildplan.

Since these are public API in jib-core that we cannot remove, this means we will have to maintain duplicate code between jib-core and jib-container-build-plan, hopefully for a short period of time. I imagine that once jib-core is adjusted to depend on jib-container-build-plan, these duplicated classes in jib-core may become a thin proxy to the corresponding classes in jib-container-build-plan. For example, AbsoluteUnixPath in jib-core could merely delegating calls to AbsoluteUnixPath in jib-container-build-plan.

I wish I could just move these classes and remove them from jib-core, but I think we want some stability in the jib-core API. Let me know if you have better ideas.

[chanseokoh]

To get the idea of actual classes to be added, see this comparison. Still working on it, but it's close to completion.

[loosebazooka]

I think this is fine. maybe jib-*container*-build-plan is redundant? jib-build-plan might be enough. We can of course revisit naming later, so feel free to submit how you want.