[Go] [Breaking Change] `genkit.GenerateData` should return `Out` not `*Out`

**Describe the bug**
`genkit.GenerateData` returns `*Out` instead of `Out` which means users don't get a choice in returning a value instead of a pointer. The rest of the structured data API like `genkit.GenerateDataStream` and `DataPrompt.Execute` already return the type, not its pointer.
