Tracking issue for plan chain rule support

Tracks progress towards supporting plan chain rules. This requires supporting adjoints of plans, and that in turn requires that we create a package for testing FFT plans to give downstream implementations a way of easily testing adjoint functionality.

- [x] Implement adjoints of plans (#67)
- [x] Create a TestUtils submodule and add adjoint plan testing (#78)
- [x] QOL improvements to adjoint styles (https://github.com/JuliaMath/AbstractFFTs.jl/pull/109)
- [ ] Implement adjoint traits in downstream packages (FFTW, CUDA, AMD, PencilFFTs) and test using TestUtils (https://github.com/JuliaMath/FFTW.jl/pull/249)
- [ ] drop Zygote FFT plan rules (https://github.com/FluxML/Zygote.jl/pull/1437)

Additionally, for full support of FFTW's additional transforms
- [] Adjoint support for R2R plans
- [] DCT chain rules (https://github.com/JuliaMath/FFTW.jl/pull/273)

And #112 needs resolution at some point.