Add support for `Q_CLASSINFO` on qobjects

Likely add support for a #[q_classinfo("name", "value")] attribute on the qobject (similar to #[qml_element = "name"], this would need the following changes.

• Add q_classinfo to allowed attrs

https://github.com/KDAB/cxx-qt/blob/2cb6df179b37e760878413d480edd623c61cfdd7/crates/cxx-qt-gen/src/parser/qobject.rs#L50-L61

• Likely add parsing of the q_classinfo(name, value) to a similar method to parse_qml_metadata, but likely parse_metadata (as this is not limited to only QML, likely have a map of values stored in the ParsedQObject

https://github.com/KDAB/cxx-qt/blob/2cb6df179b37e760878413d480edd623c61cfdd7/crates/cxx-qt-gen/src/parser/qobject.rs#L132-L150

• Add generation from the map of values to the C++ generation to add Q_CLASSINFO(name, value) to the vector of metaobjects

https://github.com/KDAB/cxx-qt/blob/2cb6df179b37e760878413d480edd623c61cfdd7/crates/cxx-qt-gen/src/generator/cpp/qobject.rs#L63-L83

• Add extra unit tests to cover the cases

• Add documentation to the extern "RustQt" book page

Discussed in https://github.com/KDAB/cxx-qt/discussions/1447