[Go] Add `DefineSchema` and add support for referencing in Dotprompt

[apascal07]

Describe the bug
You should be able to define a schema in code and let it be referenced in Dotprompt files instead of defining an inline schema.

[ihan211]

Please confirm my understanding before implementation:

1. In genkit, add a func DefineSchema(r *registry.Registry, name string, structType any) error{...}
2. In dotprompt, add a method (dp *Dotprompt) ResolveSchemaReferences(meta PromptMetadata) (PromptMetadata, error) {...}

[apascal07]

Yes that sounds roughly correct.