Schema improvement - possibly implemented using cue's struct disjunction

[trumant]

The spec documentation hints at a constraint that is not currently implemented in schema.cue

The schema declares header.project-si-source and project as optional, however, the documentation for project explains This field is not required if header.project-si-source is supplied.

https://cuelang.org/docs/tour/types/sumstruct/ provides an example of how we might accomplish the schema changes