Testing the nettle library to sign messages.

Use HMAC SHA256 on messages, with the "signature" previously padded with
zeros. The signature is then encoded in hexadecimal.
Then the signature is then replaced in-place within the message.
(I.e., without reusing a json library since some library may reorder
json parameters or add spaces.)

The key is computed from a passphrase using PBKDF2 HMAC SHA256 with 4096
iterations and the null-terminated string "xAAL" as salt.

A timestamp is added. A typical acceptance window is of 5 seconds.