Parsing the trust engines

[Rod Widdowson]

Another relying-party.xml question.  Mostly for Scott this time.

Inside this file we meet several <sec:TrustEngine> definitions.  References
to these are shared between the <SecurityPolicy> statements and the
<SignatureValidation> metadata filter.

Again, as discussed last week, we do not believe that the <SecurityPolicy>
statements are meaningful since they are superseded by the contents of
security-beans.xml.  Further it seems to me that as a result any beans
defined inside relying-party.xml will not be available to the actions
without explicit work to make them visible (because they are defined in a
child context).

Thus it seems to me that the only sensible use of a <sec:TrustEngine> inside
relying-party.xml is as (legacy) input to <metadata:SignatureValidation>.
Unfortunately it doesn't mean that I don't have to be able to parse all
variants since I'm guessing that it's perfectly acceptable to have
arbitrarily complex trust engines associated with a metadata filter.

OTOH if we do want to define all trust engines inside relying-party.xml then
it should not be too difficult to teach the beans inside security-beans.xml
to lookup up trust engines via a Service.  However I will note that in V2
these trust engines are all contained below the <!-- DO NOT EDIT BELOW THIS
POINT --> mark.

Rod