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>