tls13/key_update: Expose a manual KeyUpdate function

tls13/key_update: Expose a manual KeyUpdate function

As part of supporting KeyUpdate in ktls-utils and NVMe-OF we need to
trigger an update of the local keys after the kernel has received a
KeyUpdate message.

This patch creates a new gnutls_handshake_update_receiving_key() function
that allows updating the local keys without sending any KeyUpdate
requests.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Modified-by: Daiki Ueno <ueno@gnu.org>