enable functions such as cols_label() and cols_align() for gt_grp objects

[shannonhaughton]

Prework

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• Search for duplicates among the existing issues (both open and closed).

Proposal

Currently functions such as cols_label() and cols_align have a stop_if_not_gt_tbl(data = .data), and wont work with a gt_grp. A workaround is to map over each table and apply the function to the individual gt_tbls. Propose a function which does this looping for us.

Reprex

`
gt_tbl <- gt::exibble |>
gt()

gt_grp <- gt_group(gt_tbl, gt_tbl) |>
cols_align(
align = "center"
)

`