Adds support for service account impersonation when a URL for service account impersonation is provided.
Change-Id: I9f3bbd6926212cecb13938fc5dac358ba56855b8
GitHub-Last-Rev: 9c218789db
GitHub-Pull-Request: golang/oauth2#468
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/285012
Run-TryBot: Cody Oss <codyoss@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Cody Oss <codyoss@google.com>
Trust: Tyler Bui-Palsulich <tbp@google.com>
Reviewed-by: Cody Oss <codyoss@google.com>