[manifest] Add Containerizer, a manifest builder
[manifest] Add Containerizer, a manifest builder

Containerizer is a type for building a manifest container with
particular data and signing it, while writing it to a pre-existing
buffer. This type will be useful for tools that need to actually create
and sign such manifests.

Signed-off-by: Miguel Young de la Sota <71b8e7f4945fd97b98544cf897992af89646547a@google.com>

lowRISC/manticore,lowRISC/manticore,lowRISC/manticore