XXXXFilesystem types in the secuirty schema.

[Rod Widdowson]

> If it's trivial to do Resource support, my inclination would be to create
> new elements called PKIXResource, etc. and do it that way and just
> deprecate these.

I thought of that and initially rejected it because the element types exist
already - although they were never implemented in V2.

On further thought this would make a lot of sense since this could just be
an implementation of the old schema type.

The only possible gotcha is that in other places (well at the
ResourceBackedMetadataProvider) Resource refers to the old style <Resource>
elements whereas here we are referring to a (thinly disguised) Spring
resource.  I'm quite relaxed about this.  All uses of resource based
anything (metadata, credentials or trust) will be edge cases; but I wanted
to get the issue aired.

Rod