Add `required` contraints to schema objects

[ppkarwasz]

Many schema objects in the OpenAPI specification do not define required constraints for their most significant fields. Examples include identifier, collection, collection-update-reason, artifact, and checksum.

Additionally, some schemas declare required properties that do not exist, such as in component-release.

[taleodor]

I added to artifact in #215 , separate PR(s) needed for the rest.