fix(dht): use new DHKE shared secret type

[AaronFeickert]

Description

Ensures safer use of ECDH shared secrets by switching to the new DiffieHellmanSharedSecret type. Updates tari-crypto to v0.15.7 to accomplish this.

Motivation and Context

Currently, an ECDH secret used for message keys is produced as a RistrettoPublicKey, converted to bytes, and returned as a byte array. However, neither the RistrettoPublicKey nor the byte array are cleared when dropped. In conjunction with tari-crypto PR 137, this work ensures both the RistrettoPublicKey and byte array representations of the ECDH secret are zeroized on drop by using that PR's new DiffieHellmanSharedSecret type.

How Has This Been Tested?

Tested after applying tari-crypto PR 137, which adds the new DiffieHellmanSharedSecret generic type.

[sdbondi]

ACK

[hansieodendaal]

utACK

[AaronFeickert]

Now updates tari-crypto to v0.15.7 and is safe to merge once CI passes.

[AaronFeickert]

This is currently blocked on tari-crypto issue 142, which addresses the underlying CI failure.

[sdbondi]

@AaronFeickert Try adding the tari_crypto/wasm feature

base_layer/key_manager/Cargo.toml

- wasm = ["wasm-bindgen", "js", "tari_common_types", "console_error_panic_hook"]
+ wasm = ["tari_crypto/wasm", "wasm-bindgen", "js", "tari_common_types", "console_error_panic_hook"]

[stringhandler]

I had confused this with another issue. This is not network breaking