cmd/go: add GOAUTH mechanism for HTTP authentication
This change adds a new environment variable GOAUTH which takes a semicolon-separated list of commands to run for authentication during go-import resolution and HTTPS module mirror protocol interactions.
This CL only supports netrc and off. Future CLs to follow will extend support to git and a custom authenticator command.
For #26232
Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest,gotip-windows-amd64-longtest
Change-Id: I6cfa4c89fd27a7a4e7d25c8713d191dc82b7e28a
Reviewed-on: https://go-review.googlesource.com/c/go/+/605256
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Michael Matloob <matloob@golang.org> Reviewed-by: Alan Donovan <adonovan@google.com>