🐛 [ComboBox] handle `ComboBoxItem.child` height

If `ComboBoxItem.child` contains a Widget that takes more than the default one-row height, it overflows.
So, for instance, if we want to add a caption or subtitle in each `ComboBoxItem`, we currently can't.

This is what it [may look like](https://react.fluentui.dev/?path=/docs/components-combobox--docs#complex-options)

It's another subject, but it could be handy to handle separators and grouping like [this](https://react.fluentui.dev/?path=/docs/components-combobox--docs#custom-options) also :

Locate the ComboBoxItem implementation and its layout or height handling, then reproduce an item whose child contains a taller widget such as a caption or subtitle. Check how the default one-row height causes overflow. Done means taller children fit without overflow while ordinary items retain their expected height.

Written by the indexing model from the issue text.