Implement the Union Shape

[jderusse]

fix #1900

This PR add support for union shape.

• Use abstract class that return null for all membres
• For each member, add a TheParentMemberTheChild class
• Child class extends provide a single not null property (parent provides the rest of the props)
• Populate methods returns the right child and throws exception when the child is unknown (all members are null)
• static create methods return the right child and throws exception if the child is unknown

[stof]

• static create methods return the right child and throws exception if the child is unknown

this would mean we throw an exception if the API answers with a new value in the union, which is something that AWS does not treat as a BC break.
The official AWS SDKs instead of a special member in the union UNKOWN_TO_SDK (the actual name differs between languages but they have such concept).

• Use abstract class that return null for all membres

I don't think having all nullable properties is a good idea. It makes the SDK types less precise for dubious benefits. See my comment on the issue showing that your "simple" code justifying this choice is actually buggy code (which won't be detectable by static analysis in this solution, while it could be detected in others)

[stof]

If you want the convenience API for cases with known types, maybe we should follow the lead of the Kotlin SDK.

[jderusse]

The last remaining thing is deciding whether "abstract class returns null by default or not"

I need time to process this, and check if makes sense or not.

If you want the convenience API for cases with known types, maybe we should follow the lead of the Kotlin SDK.

I'm not sure to understand what you mean by this?
edit

[jderusse]

after looking deeper, indeed, having null is a fortunate edge case for RDS results because null is a possible value of the union.

I've removed it from this PR