Remove discriminator fields from union types #1397

We should update the style guide (and the API validation guide) and clarify how to handle union types.

• Don't use proto oneof.
• Each union member is wrapped into a message. If it's a primitive type, it should be proto optional.
• No discriminator field.
• Validate using k8s:unionMember.

This task is to update the style guide and make sure the code adheres to it. I think the only case where we have discriminator field today is in ActorTemplate.volume (field type).