Allow .cols_align() to set vertical alignment

[s-banach]

Prework

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

Proposal

When I generate a table, I see the css .gt_row { ...vertical-align: middle; ...}.
I believe there's currently no way to change this?
The .cols_align() method only seems to handle horizontal alignment.