What problem does this feature solve?

It feels natural to have an expand all button alongside with onExpandAll property, the same way it's done for selection column. It can be achieved using existing api, but UI would be much cleaner with header button.

And perhaps it makes sense to group expandable options too. (see proposal api).

p.s. Thanks for the amazing product!

What does the proposed API look like?

<Table
        ...
        rowExpansion={{
          expandedRowKeys: <...>,
          expandedRowRender: <...>,
         ...


          onExpandAll: <..>,
        }}
/>