Golang cluster specifier

The HTTP Golang cluster specifier allows Golang to choose router cluster and makes it easier to extend Envoy.

Go cluster specifier plugins can be recompiled independently of Envoy.

Warning

The Envoy Golang cluster specifier is designed to be run with the GODEBUG=cgocheck=0 environment variable set.

This disables the cgo pointer check.

Failure to set this environment variable will cause Envoy to crash!