Feature Request: Format col spanners with function

[astrowonk]

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

The function cols_label_with lets you pass in a function to change a column name, which makes it easy to turn petal_width into Petal Width for example. However, if used after tab_spanner_delim petal_width_null_count or something like that gets a column spanner of null_count with no way to format it into "Null Count"

Either letting cols_label_with apply to spanners with a kwarg, or another method to format spanner names would be super helpful.