google: add Credentials.UniverseDomain to support TPC

Read and expose universe_domain from service account JSON files in
CredentialsFromJSONWithParams to support TPC in 1p clients.

Change-Id: I3518a0ec8be5ff7235b946cffd88b26ac8d303cf
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/531715
Run-TryBot: Cody Oss <codyoss@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cody Oss <codyoss@google.com>

google/default_test.go

@@ -0,0 +1,43 @@
+// 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 google
+
+import (
+	"context"
+	"testing"
+)
+
+var jwtJSONKeyUniverseDomain = []byte(`{
+  "type": "service_account",
+  "project_id": "fake_project",
+  "universe_domain": "example.com",
+  "private_key_id": "268f54e43a1af97cfc71731688434f45aca15c8b",
+  "private_key": "super secret key",
+  "client_email": "gopher@developer.gserviceaccount.com",
+  "client_id": "gopher.apps.googleusercontent.com",
+  "auth_uri": "https://accounts.google.com/o/oauth2/auth",
+  "token_uri": "https://oauth2.googleapis.com/token",
+  "auth_provider_x509_cert_url": "https://www.googleapis.com/oauth2/v1/certs",
+  "client_x509_cert_url": "https://www.googleapis.com/robot/v1/metadata/x509/gopher%40fake_project.iam.gserviceaccount.com"
+}`)
+
+func TestCredentialsFromJSONWithParams_UniverseDomain(t *testing.T) {
+	ctx := context.Background()
+	scope := "https://www.googleapis.com/auth/cloud-platform"
+	params := CredentialsParams{
+		Scopes: []string{scope},
+	}
+	creds, err := CredentialsFromJSONWithParams(ctx, jwtJSONKeyUniverseDomain, params)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if want := "fake_project"; creds.ProjectID != want {
+		t.Fatalf("got %q, want %q", creds.ProjectID, want)
+	}
+	if want := "example.com"; creds.UniverseDomain() != want {
+		t.Fatalf("got %q, want %q", creds.UniverseDomain(), want)
+	}
+}