Allow layer_residual_quantiles() to interact with any keys

[dajmcdon]

• The current version on the frosting branch produces residuals ignoring all the keys.
• The updated version in Intro vignette #116 groups the residuals by key (all of them) before calculating the quantiles.

The update also has an argument in arx_args_list() and flatline_args_list() that determines whether to group by key. But it's currently ignored in #116.

• Modify the two *_args_list() functions to take some sort of selectors. Do we want these grouped by 1 key, all keys, some subset? The default should be "all", but we should let the user choose more than just "none" (current version) or "all" (updated version in Intro vignette #116)
• Modify the slather.residual_quantiles() to correctly handle all the cases (no grouping, group_by selected keys, group_by all keys).

[dajmcdon]

Closed by #116