Envoy's L4 Golang extension

Recently, my team implemented the [L7 golang extension](https://github.com/envoyproxy/envoy/pull/22573). Similarly, we have implemented an L4 golang extension using the same cgo technique as L7. And the L4 extension has already been widely utilized within our company for almost a year, enabling our internal customized protocol to benefit from the high-performance network infrastructure of Envoy and the superior development efficiency of Golang. Additionally, the L4 golang extension can also provide a better implementation path for envoy's future support of Dapr, etc. Given the above, we are consider to open source our L4 golang extension very soon.