feat: Introduce Exa.TrkX plugin

[benjaminhuth]

This PR introduces the Exa.TrkX plugin which implements the Exa.TrkX GNN-based track finding alongside with some examples.

I have already added a section to the CI-workflow-description, which won't work until the required docker image has been merged to the machines-repository (see acts-project/machines#43)

Points which may need some discussion are

• the cmake configuration for CUDA, which I moved to a macro since it is now needed for two plugins.
• wether we want to put ONNX model files into the repository (at the moment there are no working ones due to a bug in ONNX I think)

[benjaminhuth]

@paulgessinger Now where the first attempt to get the GPU CI running failed, should we merge this in and add the runtime-test later, or should we still wait?

[paulgessinger]

Yes, let's do that. No point in dragging this out longer for now.

[xju2]

@paulgessinger Now where the first attempt to get the GPU CI running failed, should we merge this in and add the runtime-test later, or should we still wait?

Once we merged this version, we can improve the runtime tests. We have implemented a new C++ version of the pipeline which should give us correct GNN results. It would be nice to merge those changes now and we will improve the implementation.

[paulgessinger]

I think I'm fine with the shape this PR is in now.