cmd/go: add user provided auth mode for GOAUTH
This CL adds support for a custom authenticator as a valid GOAUTH command.
This follows the specification in
https://go.dev/issue/26232#issuecomment-
461525141
For #26232
Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest,gotip-windows-amd64-longtest
Change-Id: Id1d4b309f11eb9c7ce14793021a9d8caf3b192ff
Reviewed-on: https://go-review.googlesource.com/c/go/+/605298
Auto-Submit: Sam Thanawalla <samthanawalla@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>