add InteractionTransformer

[olivierlabayle]

This PR provides an InteractionTransformer as suggested in https://github.com/JuliaAI/MLJModels.jl/issues/476.

Currently the model has hyperparameters types:

• order::Int
• colnames::Union{Nothing, Vector{Symbol}}

The second one may not be the most appropriate and happy to get some advice on it.

Otherwise happy to get your review comments and implement any missing interface function if any.

[codecov-commenter]

Codecov Report

Merging #478 (3da589d) into dev (f880634) will increase coverage by 0.48%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev     #478      +/-   ##
==========================================
+ Coverage   76.84%   77.32%   +0.48%     
==========================================
  Files          16       16              
  Lines        1097     1116      +19     
==========================================
+ Hits          843      863      +20     
+ Misses        254      253       -1     

Impacted Files	Coverage Δ
src/MLJModels.jl	100.00% <ø> (ø)
src/builtins/Transformers.jl	96.39% <100.00%> (+0.15%)	:arrow_up:
src/init.jl	100.00% <0.00%> (ø)
src/builtins/Constant.jl	62.85% <0.00%> (ø)
src/builtins/ThresholdPredictors.jl	92.43% <0.00%> (+0.84%)	:arrow_up:

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[olivierlabayle]

Looking good.

I think I've addressed your comments, please let me know if there is anything else!

[ablaom]

Closed as merged onto dev here: https://github.com/JuliaAI/MLJModels.jl/commit/6a58242951dea0a45eebb03ba022ae0611bd2cc7