Enable ec-gpu to generate MSM and FFT code for pasta_curves

[str4d]

We want to be able to leverage GPUs in halo2, via the ec-gpu crate. This requires two changes:

• The curve implementation needs to support producing the GPU kernel for finite field arithmetic.
• halo2 needs to use GPU-aware MSM and FFT algorithms.

This issue covers the second change. We should implement the GpuEngine trait from the ec-gpu crate (behind a gpu feature flag), and then the ec-gpu-gen crate should be able to generate the necessary CUDA/OpenCL source code for MSMs and FFTs.