sparse toggling mechanic

[EmilHvitfeldt]

We need a way to toggle whether sparse features should be produced or not. the general idea is fairly simple and could be done like so:

flowchart TD
    A[Does model support sparse input?]
    AA[don't make sparse data]
    B[is there high sparsity in the data?]
    BA[make sparse data]
    BB[don't make sparse data]
    A -->|Yes| B
    A -->|No| AA
    B -->|Yes| BA
    B -->|No| BB

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the hard part is finding a good default threshold for is there high sparsity in the data?. We will investigate that in tidymodels/planning#34, tidymodels/planning#33.

We will likewise get helpers from {recipes} in tidymodels/recipes#1397 to aid in this decision. Determining the sparsity of a recipe should live in the recipes package. the sparsity calculator will be located in {sparsevctrs} r-lib/sparsevctrs#82

Another wrinkle is that this determination will be preprocessor dependent. non-recipes preprocessors are easier to handle.