Consider a generic way to support pass through attributes in cxx-qt-gen

[ahayzen-kdab]

We need to be able to pass through attributes such as

• cfg
• doc
• allow
• deny
• etc

On many different elements in the original bridge macro. We should consider if there is a more generic way of doing this in the parser and generator phases with a generic type / mapping. Also see what attributes CXX is supporting https://github.com/dtolnay/cxx/blob/547a0edcf79cca2dad3f835ba736cfb85e93b1e5/syntax/attrs.rs#L286 https://github.com/dtolnay/cxx/blob/547a0edcf79cca2dad3f835ba736cfb85e93b1e5/macro/src/derive.rs#L17