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/doc.go

@@ -57,6 +57,11 @@
 // executable-sourced credentials), please check out:
 // https://cloud.google.com/iam/docs/using-workload-identity-federation#oidc
 //
+// Note that this library does not perform any validation on the token_url, token_info_url,
+// or service_account_impersonation_url fields of the credential configuration.
+// It is not recommended to use a credential configuration that you did not generate with
+// the gcloud CLI unless you verify that the URL fields point to a googleapis.com domain.
+//
 // # Credentials
 //
 // The Credentials type represents Google credentials, including Application Default
@@ -81,4 +86,5 @@
 // same as the one obtained from the oauth2.Config returned from ConfigFromJSON or
 // JWTConfigFromJSON, but the Credentials may contain additional information
 // that is useful is some circumstances.
+//
 package google // import "golang.org/x/oauth2/google"