To avoid hardcoding secret names everywhere, we want to pass them via
flags instead. As a convenience, introduce a new flag type that resolves
values of the form "secret:[project name/]<secret name>" using Secret
Manager.

This is a bit janky in the name of convenience: we need a SM client
before calling flag.Parse, which I decided should be initialized by the
user rather than implicitly. Typical usage will look like:

  var token = secret.Flag("token", "token used to do the thing")

  func main() {
    if err := secret.InitFlagSupport(context.Background()); err != nil {
      log.Fatal(err)
    }
    flag.Parse()
    fmt.Printf("My token is %v\n", *token)
  }

Supporting literal values might be unnecessary but I think it might be
helpful for local testing, and we can extend it with a file: prefix to
read from local files too.

For golang/go#51122.

Change-Id: Ie6102453c2242baf2e91b873e62e035f72a82584
Reviewed-on: https://go-review.googlesource.com/c/build/+/385185
Trust: Heschi Kreinick <heschi@google.com>
Run-TryBot: Heschi Kreinick <heschi@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Auto-Submit: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

Some of our secrets are in JSON format so that we don't have to deal
with too many. Make a little convenience flag function for those.

For golang/go#51122.

Change-Id: Ie34828443adb9acb16249339a760d28a81ddbd20
Reviewed-on: https://go-review.googlesource.com/c/build/+/386054
Trust: Heschi Kreinick <heschi@google.com>
Run-TryBot: Heschi Kreinick <heschi@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Auto-Submit: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>