Following up on WFCORE-6779...

Further refines non-Resource-based XML abstractions introduced in WFCORE-6779 for use with deployment descriptor schemas/parsers.

• Extract generalisations from XMLParticle, XMLElementReader to support both XMLElementReader and XMLAttributeReader
• Add XMLAttribute abstraction, capable of basic usage and constraint enforcement.
• Rename XMLParticleFactory -> XMLComponentFactory, as this now includes factory methods for attributes, i.e. non-particles.
• Re-orient XMLComponentFactory for use with an XMLSchema, i.e. with feature filtering and namespace resolution.
• Add unit tests validating XMLAttribute read/write semantics.

This change modifies a number of interfaces introduced in WFCORE-6779 and thus intends to target 28.0.0.Final.