To support a new type of credential: `ExternalAccountAuthorizedUser`
* Refactor the common dependency STS to a separate package.
* Adding the `externalaccountauthorizeduser` package.
Change-Id: I9b9624f912d216b67a0d31945a50f057f747710b
GitHub-Last-Rev: 6e2aaff345
GitHub-Pull-Request: golang/oauth2#671
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/531095
Reviewed-by: Leo Siracusa <leosiracusa@google.com>
Reviewed-by: Alex Eitzman <eitzman@google.com>
Run-TryBot: Cody Oss <codyoss@google.com>
Reviewed-by: Cody Oss <codyoss@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>