add freq_itemsets tunable param

[Wander03]

@kbodwin

Adds new function for min_support to dials to support adding freq_itemsets to tidyclust, see https://github.com/tidymodels/tidyclust/pull/210

[hfrick]

Thank you for the PR! Similarly to #389, I've kicked off the actions on it but I'll hold of reviewing until Emil has had a chance to look at the corresponding tidyclust PR.

[kbodwin]

Quick note on the comment on line 16 of param_min_support - we believe this param would be better using finalize, but it was out-of-scope for the project this year. It works fine now and defaults to reasonable values; just noting for a future upgrade.

[hfrick]

@kbodwin Could you turn that into an issue instead of the code comment? That makes it much easier to keep track of in the longer term.