Add border to st.columns

[jrieke]

Checklist

• I have searched the existing issues for similar feature requests.
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Summary

Add a parameter border to st.columns, similar to st.container or st.form.

Why?

We recently added optional borders to st.container. It's really nice for creating card-like layouts, e.g., for BI dashboards. These apps often show different cards with visualizations side by side through st.columns. It would make life a bit easier if we had border directly on columns, so devs don't need to wrap every card in st.container.

How?

Same API as for st.container. Show the border around each column.

Additional Context

Alternatively (or even additionally?), we could think about adding border to more elements, e.g. st.metric.

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[utkanuygur]

I think I've managed to implement the feature. Can someone review my PR?