Use case

scrollPadding exists as a parameter in TextField and was recently added to
SearchBar (#152635), but DropdownMenu doesn't expose it.

When a DropdownMenu is placed near the bottom of a scrollable view, there is
no way to control how far above the keyboard it scrolls when focused. The inner
TextField uses the default EdgeInsets.all(20.0) with no way to override it.

Current workaround

Manually listening to focus changes and calling Scrollable.ensureVisible
with a custom offset.

Proposal

DropdownMenu should have a scrollPadding parameter that passes through to
its inner TextField, consistent with TextField and SearchBar.