recursive `restrict` as pyramid

A second thought on https://github.com/JuliaImages/Images.jl/pull/971#issuecomment-910271594

gaussian_pyramid -> maybe ImageFiltering with new name build_pyramid and supports generic kernels. (EDIT @timholy: I don't think we can, gaussian_pyramid requires imresize from ImageTransformations. This might be a good candidate for remaining in Images.jl.)

It might be useful to define a more generic version of build_pyramid here with function f input.

# apply `f(restrict(img, dims))` recursively for `n_scales` times
build_pyramid(f=identity, img; dims::Dims, n_scales::Int)
# apply `f(restrict(img, dims))` recursively until `all(size(smallest_img) .< stop_size)` holds
build_pyramid(f=identity, img; stop_size)

When f(x) = imfilter(img, KernelFactors.IIRGaussian(sigma), NA()) then it becomes a gaussian pyramid.

The benefit is that we get a more generic version, and since we don't rely on either imresize or imfilter, we can keep this function in ImageBase.

FWIW, building pyramid also falls into the reduce diagram: build_pyramid(f, x::AbstractArray, n) = reduce((x,y)->push!(x, f(restrict(last(x)))), 1:n; init=[x])

Start by reading the linked ImageBase.jl discussion and the referenced ImageTransformations pull request comment, then examine the existing restrict entry point. Clarify whether build_pyramid belongs in ImageBase and which proposed signatures should be supported. Done means an agreed generic recursive pyramid API with coverage for the stated stopping conditions.