enable `MultiSelectionPrompt` and `SelectionPrompt` to show Renderables as select items.

[nils-a]

Discussed in https://github.com/spectreconsole/spectre.console/discussions/556

The original idea was sparked somewhere in https://github.com/spectreconsole/spectre.console/discussions/550: Enable MultiSelectionPrompt and SelectionPrompt to show Renderables as select items.

Currently, items for MultiSelectionPrompt and SelectionPrompt need to be string, have a TypeConverter to string or override the Converter property to produce a string.

This issue proposes to change the Converter to be able to produce a Renderable that can then be rendered as the item.