TrustEngine refactoring

[Brent Putman]

I pretty much have the refactoring done at the OpenSAML level for the
SignatureTrustEngine and also the TLS client cert X509Credential
TrustEngine stuff (OSJ-84 and OSJ-85).  I have also added a slot on the
IdP SecurityConfiguration class for the new
X509CredentialValidationConfiguration, but otherwise nothing in the IdP yet.

As-is, it's gonna break stuff in the IdP when I check it in.  Some tests
certainly, maybe some compile, not sure.  Would you like me to check-in
what I have and let it break, and we fix as necessary? If so, since
we're into the weekend, do you want me to wait until Monday or just do
it now?  Or me take a stab at fixing it all up myself in one go?  I know
some of the latter, but I don't think I know where everything is
config-wise.

The X509 stuff in particular is going to require new action(s) and/or
adding stuff to existing actions to invoke the new
X509CredentialValidationParametersResolver to populate the
X509CredentialValidationParameters on the
X509CredentialSecurityParametersContext on the MessageContext (whew).

I don't think much should be necessary for the SignatureTrustEngine
stuff other than changing the Spring wiring to put the trust engine on
the configuration instance(s) rather than wiring into the handlers.  The
existing resolver and params and context should all just work (I think).