cmd/coordinator: add CONNECT proxy support in reverse buildlet mode Updates golang/go#15072 Change-Id: I56767a421428418add66aa9b50f0baf9aa202538 Reviewed-on: https://go-review.googlesource.com/46724 Reviewed-by: Andrew Bonventre <andybons@google.com> Reviewed-by: Shawn Walker-Salas <shawn.walker@oracle.com>