rayon parallel PoW grinding for CpuBackend

[Okm165]

Parallelized Proof-of-Work Grinding Implementation

This PR introduces enhancement to the proof-of-work (PoW) grinding process in CpuBackend by leveraging the rayon crate for parallel computation when the parallel feature is enabled.

Key changes include:

• Refactored the nonce-checking logic into a closure to reduce code duplication.
• Implemented conditional compilation to select between sequential and parallel iterators (find() vs find_any()).
• Integrated parallelism via rayon::prelude::* for improved performance in multi-threaded environments.

This change aims to enhance speed in PoW computation.

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