Registering tunable() information from an extension package?

[hfrick]

To make engine arguments tunable, we currently modify the tunable() method for the corresponding model type to include a call to parsnip:::add_engine_parameters() with the info on how to match the arguments with dials objects.

More and more engines live in parsnip extension packages and making their args tunable thus requires changes to parsnip. It would be helpful if that info could be provided via the extension package without needing to modify parsnip.

parsnip already carries a "database" in form of various tibbles with fundamental information about the models which gets modified by extension packages by calls to set_engine() etc. Can the same idea be applied to the tunable information?