google/internal/externalaccount: Removed URL validation for google URLs in ADC files

Removes URL validation for token_url, service_account_impersonation_url to allow for TPC urls and adds line to the docs to warn users. See https://github.com/googleapis/google-auth-library-nodejs/pull/1517 for same change in node.js library.

Change-Id: I85fa67ee0b99deed2adb75668a1b5501851c499c
GitHub-Last-Rev: 15d7759884
GitHub-Pull-Request: golang/oauth2#627
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/465696
Reviewed-by: Cody Oss <codyoss@google.com>
Run-TryBot: Cody Oss <codyoss@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Leo Siracusa <leosiracusa@google.com>

google/internal/externalaccount/impersonate_test.go

@@ -9,7 +9,6 @@ import (
 	"io/ioutil"
 	"net/http"
 	"net/http/httptest"
-	"regexp"
 	"testing"
 )
 
@@ -114,8 +113,7 @@ func TestImpersonation(t *testing.T) {
 			defer targetServer.Close()
 			testImpersonateConfig.TokenURL = targetServer.URL
 
-			allURLs := regexp.MustCompile(".+")
-			ourTS, err := testImpersonateConfig.tokenSource(context.Background(), []*regexp.Regexp{allURLs}, []*regexp.Regexp{allURLs}, "http")
+			ourTS, err := testImpersonateConfig.tokenSource(context.Background(), "http")
 			if err != nil {
 				t.Fatalf("Failed to create TokenSource: %v", err)
 			}