Build plan adapter methods for JibContainerBuilder

[chanseokoh]

Adds internal methods for conversion between JibContainerBuilder <--> ContainerBuildPlan. I can't think of other ways to enable this conversion while not adding public methods to JibContainerBuilder.

As we currently build a JibContainerBuilder instance (which is the front-end public API) in our plugins code, the idea for the extension framework is basically like this:

1. Construct JibContainerBuilder (current behavior).
2. Convert JibContainerBuilder to ContainerBuildPlan and pass it to extensions.
3. Extensions modify the ContainerBuildPlan.
4. Convert ContainerBuildPlan back to JibContainerBuilder.
5. Plugins code proceeds with JibContainerBuilder to build an image (current behavior)

Another approach would be to revamp everything around BuildContext and ContainerConfiguration in jib-core. However, I think the amount of refactoring work to do so is prohibitively enormous, and this is not practical.