[WIP] Add quasi-3D Integrated Green Functions solver

[aeriforme]

This PR implements a quasi-3D Poisson solver based on the Integrated Green's functions. It solves the 2D Poisson equation on the (x,y) plane for every slice z. It is useful for beam-beam simulations.

See PR #4648 for the full 3D solver.

• [x] FFTW
• [x] CuFFT
• [ ] RocFFT: added but not tested - I don't know where to test it
• [x] CI test
• [x] documentation
• [ ] decide whether -DWarpX_FFT should be necessary with the 3D IGF heFFTe solver (technically it isn't)

:hocho: :fish: