This patch adds a subcommand to pull the data necessary to construct a
PlatformConfig DTO. The PlatformConfig message contains all of the
fields and values necessary to build a platform and the platform
components. This is an alternative to holos passing multiple tags to
CUE. The PlatformConfig is marshalled and passed once.
The platform config is also stored in the local filesystem in the root
directory of the platform. This enables repeated local building and
rendering without making an rpc call.
The build / render pipeline is expected to cache the PlatformConfig once
at the start of the pipeline using the pull subcommand.
