Add support for caching

[krypticmouse]

This pull request introduces a new caching mechanism for predictions, refactors some utility imports, and enhances the Prediction data structure for better interoperability. The most important changes are grouped below:

Caching Infrastructure:

• Added a new ResponseCache implementation in crates/dspy-rs/src/utils/cache.rs using the foyer and tempfile crates, providing an async hybrid in-memory and disk-backed cache for predictions. This includes a generic Cache trait for extensibility.
• Updated dependencies in crates/dspy-rs/Cargo.toml to include foyer (for caching) and tempfile (for temporary directory management).

Data Structure Improvements:

• Enhanced the Prediction struct in crates/dspy-rs/src/data/prediction.rs to support conversion from a vector of key-value pairs and iteration over its data, improving flexibility and compatibility with the caching layer.
• Added serde_json::Value import to support the new data handling in Prediction.

Utility Refactoring:

• Refactored imports in crates/dspy-rs/src/adapter/chat.rs to use serde_utils::get_iter_from_value instead of the previous location, supporting the new module structure.
• Created new utility modules cache and serde_utils in crates/dspy-rs/src/utils/mod.rs, and re-exported their contents for easier access throughout the codebase.

Closes #21