ovpn: implement packet processing

This change implements encryption/decryption and
encapsulation/decapsulation of OpenVPN packets.

Support for generic crypto state is added along with
a wrapper for the AEAD crypto kernel API.

Signed-off-by: Antonio Quartulli <antonio@openvpn.net>
Link: https://patch.msgid.link/20250415-b4-ovpn-v26-9-577f6097b964@openvpn.net


Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>