More standard time mocking using Clock

[crizzis]

SamlClient provides a way of mocking the timestamp using setInstantNow, which works just fine.

However, the standard way of mocking time in Java 8+ is to supply a Clock instance, which allows more flexibility, and eliminates the need of 'special' methods being exposed only for testing.

So, the suggestion here is perhaps SamlClient could expose setClock instead, which would be safe for calling both in production and test code.