Add shared derivation material support for thread-safe key generation, use with NM clients

[durch]

• Add serialization support to DerivationMaterial for persistence/sharing
• Implement shared derivation material in BaseClientBuilder with Arc<Mutex<>> for thread safety
• Add network monitor support for deterministic key generation from file
• Extend MixnetClientBuilder with shared derivation material functionality
• Add comprehensive documentation explaining thread-safe sharing and key generation priority

This enables multiple clients to derive keys from the same source material while maintaining thread safety, particularly useful for network monitoring where consistent client identities are needed across multiple instances.

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