aws: New package which configures oauth2 endpoints

As per discussion in #401 and gerrit I have built out the proposed endpoint package.

I migrated all the existing endpoints, not sure if you wanted this but it does illustrate the pattern.

endpoints/endpoints_test.go

@@ -0,0 +1,43 @@
+// Copyright 2019 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 endpoints
+
+import (
+	"testing"
+
+	"golang.org/x/oauth2"
+)
+
+func TestCognitoEndpoint(t *testing.T) {
+
+	var endpointTests = []struct {
+		in  string
+		out oauth2.Endpoint
+	}{
+		{
+			in: "https://testing.auth.us-east-1.amazoncognito.com",
+			out: oauth2.Endpoint{
+				AuthURL:  "https://testing.auth.us-east-1.amazoncognito.com/oauth2/authorize",
+				TokenURL: "https://testing.auth.us-east-1.amazoncognito.com/oauth2/token",
+			},
+		},
+		{
+			in: "https://testing.auth.us-east-1.amazoncognito.com/",
+			out: oauth2.Endpoint{
+				AuthURL:  "https://testing.auth.us-east-1.amazoncognito.com/oauth2/authorize",
+				TokenURL: "https://testing.auth.us-east-1.amazoncognito.com/oauth2/token",
+			},
+		},
+	}
+
+	for _, tt := range endpointTests {
+		t.Run(tt.in, func(t *testing.T) {
+			endpoint := Cognito(tt.in)
+			if endpoint != tt.out {
+				t.Errorf("got %q, want %q", endpoint, tt.out)
+			}
+		})
+	}
+}