spotify: add Spotify endpoints

Added Spotify endpoints as per documentation: https://developer.spotify.com/web-api/authorization-guide/

Fixes golang/go#23338

Change-Id: Ic1a0d6c8f35923b1c6d497e07f028fe92e97e6b6
Reviewed-on: https://go-review.googlesource.com/86250
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: FenwickElliott

spotify/spotify.go

@@ -0,0 +1,16 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package spotify provides constants for using OAuth2 to access Spotify.
+package spotify // import "golang.org/x/oauth2/spotify"
+
+import (
+	"golang.org/x/oauth2"
+)
+
+// Endpoint is Spotify's OAuth 2.0 endpoint.
+var Endpoint = oauth2.Endpoint{
+	AuthURL:  "https://accounts.spotify.com/authorize",
+	TokenURL: "https://accounts.spotify.com/api/token",
+}