Implement jsonformer

[itsbalamurali]

Implement jsonformer: https://github.com/1rgs/jsonformer

[philpax]

Hi, thanks for requesting this! This is a duplicate of #235 (which is a more general issue), so I'm going to close this, but feel free to vote on that issue to prioritise it :)

[pixelspark]

I have an implementation of this using llm, basically it is a dyn Sampler you plug in. Can publish as a separate crate (and imho should not be part of llm core, although I like the idea in #235 of providing more hooks for guided generation).