openssl: add kTLS support option

openssl: add kTLS support option

This commit add option to enable kTLS support, improving
performance by offloading TLS encryption and decryption to
kernel space.

- Reduced CPU usage by minimizing data copying between user space
  and kernel space.
- Enables the use of the sendfile() system call with encrypted
  sockets for zero-copy data transmission.
- Leverages hardware-accelerated NIC that support TLS offloading.

Signed-off-by: Tan Zien <nabsdh9@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/21306
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>