better types in lss message

[Evanfeenstra]

server_hmac should be Option<[u8; 32]>. Because an hmac is not created if the mutations are empty, so it could be None (right now if there is no hmac, then its a zero-length array, but that is confusing and was a mistake on my part)

• signer/handle_lss_msg should fail if muts are NOT empty, but server_hmac is None https://github.com/stakwork/sphinx-rs/blob/master/lss-connector/src/signer.rs#L168
• check_hmac should return false if server_hmac is None (for example in signer/handle_lss_msg Msg::Created, check_hmac is only called if muts are not empty) https://github.com/stakwork/sphinx-rs/blob/master/lss-connector/src/signer.rs#L121
• in other places we'll need to do try_into()? to convert Vec to [u8; 32]
• serializers should handle the Option, and tests should include both Some and None for server_hmac

client_hmac should be [u8; 32]

auth_token should be [u8; 32]