This adds a non-binding, documentational-only json-schema to
schemas/osbuild1.json which describes the format of the pipeline
manifest taken as input to osbuild. This is currently for
documentational purposes, but is definitely open to be used for actual
runtime verification.
The manifest does not describe options to assemblers, stages, or
sources. These are left as arbitrary json-objects and need separate
validation, if required. Note that most stages already contain an
embedded schema for their parameters.
