Add blockConcurrencyWhile binding for Durable Objects

[lukevalenta]

NOTE: I used AI to help draft this PR, but I understand and approve the code and think this would be a useful feature for the Rust bindings.

Bind the runtime's blockConcurrencyWhile via State::block_concurrency_while (errors reset the object) and block_concurrency_while_infallible (errors returned as values). This mostly addresses #177, but since a literal async constructor isn't supported we instead document a lazy-init handler pattern.

[codspeed-hq]

Merging this PR will not alter performance

⚠️ Unknown Walltime execution environment detected

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✅ 2 untouched benchmarks

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_{Comparing lukevalenta:lvalenta/do-async-init (9188a17) with main (5f2d6c9)}