feat: tool response format - string vs schema

[grabbou]

          btw. @grabbou it's not exactly about this line, but - shouldn't we allow tools to respond with proper (and well-defined using `zod`) objects (JSON serializable)? We still could serialize it to messages stack but making it so first - make the tools more reusable (and even composable/stackable), less error-prone - think about it, I think we should.

Originally posted by @pkarw in #44 (comment)