Worst case, you can implement this interface yourself now easier than ever. Since golang is statically linked and has super fast startup times, you can build an interface where Lambda launches your golang process, and that process binds to a port and spawns a sub-process that implements the actual Lambda function. The golang process would implement the "socket" based interface for you.
People have been doing the above already, but all the existing runtimes, including Node.js, Java, Python, and .NET, have non-trivial startup times and memory usage compared to a golang binary, which should be able to start up in 10s of milliseconds and use less than 1 MB of memory while acting as a proxy.