Tracking issue for plan chain rule support

[gaurav-arya]

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.

• Implement adjoints of plans (Chain rules for FFT plans via AdjointPlans #67)
• Create a TestUtils submodule and add adjoint plan testing (Put tests of FFT backends into TestUtils submodule  #78)
• QOL improvements to adjoint styles (Rename ProjectionStyle -> AdjointStyle, _mul -> AdjointMul, and improve docs #109)
• Implement adjoint traits in downstream packages (FFTW, CUDA, AMD, PencilFFTs) and test using TestUtils (Support new AbstractFFTs.AdjointStyle trait for FFT and DCT plans FFTW.jl#249)
• drop Zygote FFT plan rules (Remove Zygote rule for *(::AbstractFFTs.Plan, ::AbstractArray) FluxML/Zygote.jl#1437)

Additionally, for full support of FFTW's additional transforms

• [] Adjoint support for R2R plans
• [] DCT chain rules (chain rules for DCT FFTW.jl#273)

And #112 needs resolution at some point.

[vpuri3]

Also JuliaMath/FFTW.jl#273