cmd/coordinator: add CONNECT proxy support in reverse buildlet mode
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 <682ad9f487f614f19e8b7cc5d340bdc8e1c1b128@google.com>
Reviewed-by: Shawn Walker-Salas <5ad8b1618966d624d2c82ee134e1e7e8ef0fd1d6@oracle.com>

golang/build,golang/build,golang/build,golang/build