Improve the group header formatting and rendering options.

[simonssspirit]

Improve the group header formatting and rendering options.

Currently, the group header will render the value as it is, which can cause unexpected results with dates, boolean or null values.

We can think of a safe way to allow easier formating and general rendering customization of the group header.

[mcapkovic]

also what was confusing to me is that group cells are using GridCellProps but the "format" prop is always undefined (even with GridColumn "format" prop populated)

[simonssspirit]

reported again in 1508575