Introduce NsgaiiBuilder and TpeBuilder for samplers

[Alnusjaponica]

Summary

Replace the XxxConfig/from_config construction pattern with builders following the API style of std::thread::Builder: settings are configured by chaining setter methods and the sampler is created with build.

let sampler = NSGAIISampler::builder()
    .population_size(100)
    .mutation_prob(0.1)
    .crossover_prob(0.9)
    .swapping_prob(0.5)
    .seed(42)
    .build();

let sampler = TpeSampler::builder()
    .n_startup_trials(20)
    .multivariate(true)
    .seed(42)
    .build();

Changes

• Add NsgaiiBuilder and TpeBuilder (with Default) exposing builder(), chainable setters, and build()
• Remove NsgaiiConfig/TpeConfig and from_config
• Keep new, seed_from_u64, and Default on the samplers for backward compatibility; they now delegate to the builders
• Update the pyo3 bindings to construct samplers through the builders