fix(k1util): zeroize secret key buffers in load and save

[varex83agent]

Closes #111.

k1util::load and k1util::save moved the raw secp256k1 secret through
intermediate buffers — the hex string read from disk and the decoded scalar in
load, the serialized scalar and its hex encoding in save — that were dropped
without being wiped. Each is now held in zeroize::Zeroizing so it is cleared on
drop, matching the existing precedent in crates/crypto and crates/eth2util.

Behaviour, file contents and the 0o600 permissions are unchanged; this is
defence-in-depth only. Charon's Go Load/Save do not zeroize (Go offers no
equivalent guarantee), so this is a Rust-side hardening rather than a parity fix.

Co-Authored-By: Bohdan Ohorodnii 35969035+varex83@users.noreply.github.com