Support bind mounts for image building workspace

[scottfrederick]

When building an image using the Maven or Gradle plugin, the CNB builder is provided a volume mount that is used to upload the application contents and that buildpacks can use to store ephemeral data. Following on from #28387, users should be able to specify the location of this workspace as a bind mount instead of a volume. The format of the workspace configuration should match the format of the existing cache configuration specified in #28387.