Fixes for UE5.6

[hyfan1116]

Add missing header and reorganize ROSIntegrationCore for building with UE5.6

[andreschreiber]

This branch seems super useful! However, I was unable to build; seems like the _Impl header isn't present in the commit.

[andreschreiber]

I edited your code a little to get it building on Windows 11 -- I haven't tested it in a ROS system overall, but it seems to fix the import errors if the code is modified to here: https://github.com/andreschreiber/ROSIntegration/tree/ue5.6

[hyfan1116]

@andreschreiber you are right - added it in the commit. I was able to run pub / sub examples following the README on Ubuntu 22.

[Sanic]

Thanks for the PR!