feat(age): add BIP39 mnemonic support for Identity

[RyderFreeman4Logos]

Summary

This PR introduces support for BIP39 mnemonics to the age crate, enabling users to backup and restore x25519::Identity using a 24-word phrase. This facilitates deterministic key generation and easier key management.

Changes

age Crate

• Added Identity::from_secret_bytes: A public method to construct an Identity from a raw 32-byte array.
  • Note: Added a strong # Security section to the documentation warning users to only provide high-entropy inputs (CSPRNG or KDF derived).
• Added bip39 Feature:
  • Introduced an optional dependency on bip39 (v2.0).
  • Added a bip39 feature flag to Cargo.toml.
• BIP39 Implementation:
  • Implemented Identity::to_mnemonic(&self) -> Mnemonic: Serializes the secret key into a 24-word English mnemonic.
  • Implemented Identity::from_mnemonic(&Mnemonic) -> Result<Self, &'static str>: Restores an identity from a mnemonic.
  • Security: Intermediate entropy vectors are explicitly zeroized using the Zeroize trait before being dropped.

Testing

• Added unit tests for the new functionality:
  • identity_from_secret_bytes: Verifies raw byte deserialization.
  • mnemonic_round_trip: Verifies that an identity can be converted to a mnemonic and restored back correctly (guarded by bip39 feature).
  • invalid_mnemonic_length: Ensures that mnemonics with insufficient entropy (e.g., 12 words/128 bits) are rejected safely.

Verification

Run the tests including the new feature:

cargo test --package age --lib --features bip39