endpoints: add links/provider for Discord

Endpoints are provided from
https://discord.com/developers/docs/topics/oauth2#shared-resources-oauth2-urls.

Change-Id: I7bcc8b4cb5527959acb8f177ffb8bbafbe727e82
GitHub-Last-Rev: 6869fc3
GitHub-Pull-Request: #768
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/649115
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

endpoints/endpoints.go

@@ -35,6 +35,12 @@ var Cern = oauth2.Endpoint{
 	TokenURL: "https://oauth.web.cern.ch/OAuth/Token",
 }
 
+// Discord is the endpoint for Discord.
+var Discord = oauth2.Endpoint{
+	AuthURL:  "https://discord.com/oauth2/authorize",
+	TokenURL: "https://discord.com/api/oauth2/token",
+}
+
 // Facebook is the endpoint for Facebook.
 var Facebook = oauth2.Endpoint{
 	AuthURL:  "https://www.facebook.com/v3.2/dialog/oauth",