Add gemstone auth hash signer

[0xh3rman]

for gemwalletcom/wallet#390

[gemini-code-assist]

Code Review

This pull request introduces a new function sign_auth_message_hash in gemstone/src/auth.rs to sign an authentication message hash using a private key, along with corresponding unit tests. Feedback highlights a security concern where an early return on an invalid hash length prevents the private key from being wrapped in Zeroizing, potentially leaving sensitive key material in memory. It is recommended to wrap the private key in Zeroizing at the very beginning of the function and validate its length.