Add str support to dropdown

[AjayThorve]

This small PR adds str column support to drop_down and multi_select widgets, along with charts.bar, which already supports str columns directly since 23.08

import cudf
from cuxfilter import charts, DataFrame

cux_df = DataFrame.from_dataframe(cudf.DataFrame({'val': ['A', 'B', 'C', 'D', 'E']}))
multi_select = charts.multi_select('val')

d = cux_df.dashboard([multi_select])
# View the individual multi_select chart part of the dashboard d
multi_select.view(height=200)

cc @exactlyallan

[AjayThorve]

/merge