feat: add Objective trait and unify optimize API
- Add `Objective<V>` trait with lifecycle hooks (`before_trial`,
`after_trial`, `max_retries`) in new `src/objective.rs`
- Replace 14+ optimize variants with 6 methods: `optimize`,
`optimize_with`, and async/parallel counterparts
- `optimize*` methods accept closures directly (FnMut for sync, Fn for
async); `optimize_with*` methods accept `impl Objective<V>` for
struct-based objectives with hooks and retries
- Remove `optimize_until`, `optimize_with_callback`,
`optimize_with_retries`, `optimize_with_checkpoint`, and all
deprecated `_with_sampler` methods
Manuel Raimann
964b4d5749