Add kTLS support to offload TLS encryption to the kernel

[ahmadbelb]

Problem

Valkey's TLS write path goes through OpenSSL in userspace, requiring buffer concatenation in connWritev() and preventing the use of io_uring for batched writes.

[zuiderkwast]

I don't know if io_uring is likely to be prioritized soon (though there are PRs like #112), partly because it seems to be blocked in container runtimes. But there are other wins, things that would only work without TLS otherwise:

• writev() without concantenation, as you mentioned
• Psync Zero-Copy Transmission #1335 / Use MSG_ZEROCOPY for plaintext replication traffic #1543
• Possibility to offload encryption to hardware, for NICs that support it