feature: add validators

[thomas-fossati]

Currently it is left as an exercise for the user to make sure the assembled (Co)SWID is syntactically correct, e.g., that all non-optional fields have been correctly filled in and that any co-constraint is satisfied.

Ideally, the library should provide this facility via a public Valid() error function that is implemented by each component and can be recursively called by the top-level SoftwareIdentity.