Support v1.0 SLSA provenance format

[asraa]

Description of the feature request:

Support v1.0 SLSA provenance, as defined in a draft here: slsa-framework/slsa#525

This will be merged soon, and so I can help to start a PR defining the struct, so long as the type is defined with a ?draft URI for now. (Or we can wait to merge it).

One item to consider here that I've always wondered is should we be using generated Go code against the protobuf message defining the predicate? This would mean that the oneof support comes out of the box. On the other hand, there's some overhead of bringing in the proto runtime dependencies and complexity.

The oneof could also be handled by an interface, but this is also a little bit of overhead.

cc @MarkLodato @rbehjati @laurentsimon