Finalizing TrustEngine config

[Cantor, Scott]

I realized I still have a pair of basic trust engines hardwired in one of
the Spring files, and I'm loading those into the message handlers that are
doing the authentication work.

Brent, I see the SignatureValidationParameters class still has a slot for
a trust engine, but I don't think the handlers would see it, they're using
the one that's set on the handler. Plus of course there's no such
parameter set for the TLS client case.

I'd kind of prefer to set the engine using the SecurityConfiguration
somehow, because the natural place to define the engine(s) is inside the
relying-party context where the rest of the security config is done.
Should that be our goal here, or should I just find a way to define them
elsewhere?

-- Scott