[FEATURE] Allow `LlmGenerationClient::geneate()` directly return a JSON value

[badmonster0]

We may change the trait to allow LlmGenerationClient::geneate() directly return a JSON. The return type can be an enum, with either a JSON or a String, depending on if JSON schema is provided in request.

Originally posted by @badmonster0 in #395 (comment)

[par4m]

I can work on this, please assign!