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.

https://github.com/pocketbase/pocketbase/pull/3323.

Change-Id: I023871a4340ee4ebcba9fd808d4a8f0c081fc10e
GitHub-Last-Rev: 890ef056bd78d109e0ecc464263f9e85586e3f7d
GitHub-Pull-Request: golang/oauth2#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>

endpoints/endpoints.go

diff --git a/endpoints/endpoints.go b/endpoints/endpoints.go
index 950754c..0d8cf64 100644
--- a/endpoints/endpoints.go
+++ b/endpoints/endpoints.go
@@ -146,6 +146,12 @@
 	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",