endpoints: add patreon endpoint

Adds the patreon endpoints from their documentation.

https://docs.patreon.com/#oauth.

I've tested these thoroughly whilst making a pr to pocketbase.

pocketbase/pocketbase#3323.

Change-Id: I023871a4340ee4ebcba9fd808d4a8f0c081fc10e
GitHub-Last-Rev: 890ef05
GitHub-Pull-Request: #668
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/528640
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Jorropo <jorropo.pgm@gmail.com>
Auto-Submit: Jorropo <jorropo.pgm@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: ghostdevv

endpoints/endpoints.go

@@ -146,6 +146,12 @@ var Odnoklassniki = oauth2.Endpoint{
 	TokenURL: "https://api.odnoklassniki.ru/oauth/token.do",
 }
 
+// Patreon is the endpoint for Patreon.
+var Patreon = oauth2.Endpoint{
+	AuthURL:  "https://www.patreon.com/oauth2/authorize",
+	TokenURL: "https://www.patreon.com/api/oauth2/token",
+}
+
 // PayPal is the endpoint for PayPal.
 var PayPal = oauth2.Endpoint{
 	AuthURL:  "https://www.paypal.com/webapps/auth/protocol/openidconnect/v1/authorize",