Checkpoint/verify API into the language-specific SDKs

Concept:

```rust
let cp1 = golem::checkpoint("pre-json generation");

...

let json = llm.call(prompt);

// Automatically rolls back to last checkpoint if verification fails.
let result = cp1.verify(Result::from_json(json));
```

This should be implementable using the existing transaction API.

We should have the new API in:
- golem-rust
- golem-ts
