OpenAPI: Add missing buildable references

[manusa]

Description

Part of #6130

Add missing buildable references to OpenAPI Maven Plugin.

@BuildableReference(EnvVar.class),
@BuildableReference(ContainerPort.class),
@BuildableReference(Volume.class),
@BuildableReference(VolumeMount.class)

These references were added only to the Kubernetes extensions since the go model generators and annotators were different for Kubernetes and OpenShift core model types and for the extensions.

We should address this and add these references in all cases by leveraging the new model generation approach.
These seem like interesting classes that will likely improve the Sundrio builder experience.