Checkpoint/verify API into the language-specific SDKs

[vigoo]

Concept:

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-go
• golem-ts
• golem-py

[vigoo]

This should be done in the unified Rust & TS agent SDKs for 1.4