Implement secure key zeroization for cryptographic operations #12

[sandlbn]

Implement secure key zeroization using the zeroize crate to prevent sensitive cryptographic data from lingering in memory after use.

Changes:

• Add zeroize dependency with ZeroizeOnDrop for automatic cleanup
• Wrap private keys in SecurePrivateKey struct that zeroizes on drop
• Ensure all temporary buffers containing key material are cleared
• Add tests for zeroization behavior

Do not merge: Depends on IntelLabs/atlas-c2pa-lib#13

[marcelamelara]

Thanks for these changes, this LGTM! Sorry for the long delay on this PR.