Provide controls for inlining / disabling `$defs` and `$refs` in generated schema

[mattt]

As described by @belozierov in #98 (comment):

GenerationSchema is generated using $defs and $ref, but in my case the LLM API does not support these features. As a workaround, I had to recursively traverse the schema and inline all $ref references.

It would be helpful to have an option in GenerationSchema to disable the use of $ref entirely.

Additionally, if I try to manually implement static var generationSchema: GenerationSchema for @Generable type, macro-related errors occur internally.