fix(plugins): support namespaced static exports

[facontidavide]

Summary

• add backward-compatible *_PLUGIN_NAMED(ClassName, SymbolName, manifest) exports for data sources, message parsers, toolboxes, and dialogs
• keep the existing macros and generated getter names unchanged for unqualified classes
• cover all four families with a static compile/runtime test using namespaced plugin classes
• document the named form in the plugin guides and changelog

This fixes the SDK-level token-paste limitation identified while reviewing PlotJuggler/pj-official-plugins#235. Qualified class names such as mosaico::MosaicoToolbox are valid C++ types but cannot be pasted into a getter identifier; the new form separates the type from the unqualified symbol token.

No ABI or protocol change.

Tests

• ./build.sh
• ctest --test-dir build --output-on-failure — 50/50 passed