Envoy's L4 Golang extension

Recently, my team implemented the L7 golang extension. 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.