feat(XSD): add XSD libraries from macchina.io

Origin

XSD is a macchina.io platform library. As decided by the POCO team, it moves into POCO for release 2.0. The appinf and aleph-us lineages are consolidated, and the code is licensed under the POCO license (BSL-1.0).

Scope

• XSD/Types, XSD/Parser, XSD/CodeGen libraries
• XSD/Validator (aleph-us lineage, depends on libxml2 -- decide on the dependency policy)
• Testsuites

Tasks

• Import sources (library, testsuite, samples, documentation), preserving history where practical
• Adapt namespaces, include paths and POCO_* macros to POCO conventions; ASCII-only sources
• GNU make build (Makefile, components, build/config)
• CMake build (CMakeLists.txt, cmake/Poco*Config.cmake, ENABLE_XSD option)
• Testsuite runs in CI (Linux, macOS, Windows)
• PocoDoc documentation, doc/ overview page
• CHANGELOG / release notes entry for 2.0
• Reconcile divergence between the appinf and aleph-us lineages

Dependencies

Foundation, XML, Util; libxml2 for the Validator.