SAML: Configuration defaults, examples and dependencies (#17954)

* Add SAML configuration options

* Add crewjam/saml as a depdency

Needed as part of the enterprise SAML integration.

* Vendor github.com/stretchr/testify/require

The package require implements the same assertions as the `assert` package but stops test execution when a test fails.

pkg/extensions/main.go

@@ -1,8 +1,10 @@
 package extensions
 
 import (
+	_ "github.com/crewjam/saml"
 	_ "github.com/gobwas/glob"
 	_ "github.com/robfig/cron"
+	_ "github.com/stretchr/testify/require"
 	_ "gopkg.in/square/go-jose.v2"
 )