google/externalaccount: moves externalaccount package out of internal and exports it

go/programmable-auth-design for context. Adds support for user defined
 supplier methods to return subject tokens and AWS security credentials.

Change-Id: I7bc41f8c5202ae933fce516632f5049bbeb3d378
GitHub-Last-Rev: ac519b242f8315df572f1b205b0670f139bfc6c3
GitHub-Pull-Request: golang/oauth2#690
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/550835
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Leo Siracusa <leosiracusa@google.com>
Reviewed-by: Chris Smith <chrisdsmith@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cody Oss <codyoss@google.com>

google/externalaccount/header_test.go

@@ -0,0 +1,48 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package externalaccount
+
+import (
+	"runtime"
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+)
+
+func TestGoVersion(t *testing.T) {
+	testVersion := func(v string) func() string {
+		return func() string {
+			return v
+		}
+	}
+	for _, tst := range []struct {
+		v    func() string
+		want string
+	}{
+		{
+			testVersion("go1.19"),
+			"1.19.0",
+		},
+		{
+			testVersion("go1.21-20230317-RC01"),
+			"1.21.0-20230317-RC01",
+		},
+		{
+			testVersion("devel +abc1234"),
+			"abc1234",
+		},
+		{
+			testVersion("this should be unknown"),
+			versionUnknown,
+		},
+	} {
+		version = tst.v
+		got := goVersion()
+		if diff := cmp.Diff(got, tst.want); diff != "" {
+			t.Errorf("got(-),want(+):\n%s", diff)
+		}
+	}
+	version = runtime.Version
+}