Security: Memory Hardening — IDisposable & Buffer Zeroization

🛡️ Summary

This PR improves the memory safety of sensitive cryptographic material in Nethereum.
It implements the IDisposable pattern for EthECKey and enforces explicit zeroization (Array.Clear) for private key and derived key buffers throughout their lifecycle.

🔒 Security Hardening Details

1. Nethereum.Signer (EthECKey)

• IDisposable Implementation: Added Dispose() to allow explicit clearing of sensitive material immediately after use.
• Buffer Zeroization: The _privateKey byte array is now explicitly cleared in the Dispose method.
• GenerateKey Fix: Updated EthECKey.GenerateKey() to use a try/finally block, ensuring that the temporary private key buffer is cleared instantly after the EthECKey instance is created.
• Safe State Handling: Methods now verify the object's disposed state, preventing accidental access to zeroed or released memory.
• Typo Fix: Corrected a minor typo in the public API (compresseed → compressed).

2. Nethereum.KeyStore (KeyStoreCrypto)

• Decrypt Security: The Decrypt method now uses try/finally blocks to explicitly zero out derivedKey and encryptKey buffers immediately after the decryption operation is completed.
• This prevents sensitive raw keys from lingering in RAM after a KeyStore file has been decrypted.

✅ Verification Results

• Unit Tests: Full verification using new security-focused tests:
  • EthECKeySecurityTests.cs: Confirms privateKey buffer is zeroed after Dispose.
  • KeyStoreSecurityTests.cs: Confirms derivedKey is zeroed after decryption.
• Target: Verified on net8.0.

Strengthening Nethereum's resistance to memory dump analysis.