Enable remaining cols_* functions for gt_group #2012

Prework

• Read and abide by gt's code of conduct and contributing guidelines.
• Search for duplicates among the existing issues (both open and closed).

Proposal

Currently functions such as cols_width() wont work with a gt_group. Apply the functionality added in PR #2010

Reprex

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

gt_grp <- gt_group(gt_tbl, gt_tbl) |>
cols_width(
num ~ px(150)
)

`