feat: Add zeroize feature

[CCristi]

Added a new zeroize feature that enables the zeroize support for both x25519-dalek and ed25519-dalek.
Other algorithms already implement zeroize by default .

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (b3fefdb) to head (13f9a08).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #136   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
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  Files           11        11           
  Lines          531       489   -42     
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- Hits           531       489   -42     

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[Copilot]

Pull Request Overview

This PR adds a new zeroize feature to enable secure memory clearing for cryptographic keys in the x25519-dalek and ed25519-dalek libraries. This feature allows users to opt into secure memory handling for these Curve25519-based cryptographic algorithms.

• Adds a zeroize feature flag that conditionally enables zeroize support
• Uses optional dependency syntax to avoid requiring x25519 or ed25519 features to be enabled

[kigawas]

Wait for #137