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.

vendor/github.com/jonboulle/clockwork/.gitignore

@@ -0,0 +1,25 @@
+# Compiled Object files, Static and Dynamic libs (Shared Objects)
+*.o
+*.a
+*.so
+
+# Folders
+_obj
+_test
+
+# Architecture specific extensions/prefixes
+*.[568vq]
+[568vq].out
+
+*.cgo1.go
+*.cgo2.c
+_cgo_defun.c
+_cgo_gotypes.go
+_cgo_export.*
+
+_testmain.go
+
+*.exe
+*.test
+
+*.swp