Introduce builders for the storage implementations

[Alnusjaponica]

Summary

Replace the XxxStorageOptions/new_with_option(s) construction pattern with builders following the API style of std::thread::Builder: settings are configured by chaining setter methods and the storage is created with build.

let storage = InMemoryStorage::builder()
    .apply_discard(true)
    .build();

let storage = SQLite3Storage::builder(file_path)
    .apply_discard(true)
    .build()?;   // Result

let storage = JournalStorage::builder(Box::new(backend))
    .apply_discard(true)
    .build()?;   // Result

Changes

• Add InMemoryStorageBuilder, SQLite3StorageBuilder, and JournalStorageBuilder (with Default where they have no required arguments), exposing builder(), chainable setters, and build()
• Remove InMemoryStorageOptions, SQLite3StorageOptions, JournalStorageOptions and new_with_option/new_with_options
• Keep new for backward compatibility; it now delegates to the builders
• SQLite3Storage/JournalStorage/CachedStorage also keep their existing new signatures
• Update the pyo3 bindings to construct storages through the builders
• Update a doc comment in cache.rs that referenced the removed options structs

Verification

• cargo test --all-features — 275 tests pass (including new doctests)
• cargo test -p rustuna_storage --locked -- --ignored (from rustuna_pyo3) — 3 pass
• cargo clippy --locked --workspace --lib --bins --tests --examples --all-features -- -D warnings — clean
• cargo fmt --all -- --check — clean
• Python: uv run --no-sync pytest tests/ — 701 pass