Feature request: Support custom rowHeights for rows at different levels of a TreeGrid

[nstoertz]

I'm building a UI where we'd like rows at the top level of our TreeGrid to have a larger row height than nested rows.

The current rowHeights property, https://reactdatagrid.io/docs/api-reference#props-rowHeights, is not adequate to support this, as we are using live pagination and our grid can contain several thousand rows, for which we load the IDs dynamically.

A couple potential ideas for a solution:

1. Allow rowHeights to be set to a callback function that would be called with the data for a row and expected to return the height for that row
2. Allow configuring the grid with nodeRowHeights property that allowed us to specify child row heights for a given parent node. Again, this would be nice to be able to set to a callback function.