[noise] encrypted nonce / udp packet number

[David Wong]

>From the code https://github.com/private-octopus/picoquic/blob/fe434572a96143e6000351780d1a8b8582011b75/picoquic/sender.c#L409

picoquic_pn_encrypt(pn_enc, send_buffer + sample_offset, send_buffer +
/* pn_offset */ pn_offset,

with:

void picoquic_pn_encrypt(void *pn_enc, const void * iv, void *output,
const void *input, size_t len);

so iv = send_buffer + sample_offset
with sample_offset = /* header_length */ pn_offset + 4;