Kernel TLS Offload (kTLS) support in Envoy

Title: Kernel TLS Offload (kTLS) support in Envoy

Description: kTLS was added into the kernel lately and it enables high performance use cases such as using sendfile to send data. It also supports potential hardware/NIC offloads to make things even faster. It would be really nice to have this feature in Envoy also. I looked up previous issues and found ~2 issues briefly mentioning kTLS, but the discussions didn't advance further. I'm wondering whether there are plans to support this?

I thought about it and maybe we can have an IoHandle abstraction that hides away the kTLS offloading, or maybe integrate it into the transport socket... 🤔

[optional Relevant Links:]

• kTLS documentation: https://docs.kernel.org/networking/tls-offload.html
• original issue that mentioned kTLS: https://github.com/envoyproxy/envoy/issues/5620
• NGINX kTLS experiment: https://www.nginx.com/blog/improving-nginx-performance-with-kernel-tls/
• Netflix slides: http://nabstreamingsummit.com/wp-content/uploads/2022/05/2022-Streaming-Summit-Netflix.pdf