[proposal] Make DropdownButton items's alignment customizable

[Juliotati]

Use case

Currently, the default alignment for the dropdown item cannot be changed from our end unless we go in and change it from the framework ☠.

We are left with all that white space if there are items in the list that have different length in characters

Proposal

It would be good if it was changed from

alignment: AlignmentDirectional.topStart,

to

alignment: itemsAlignment ?? AlignmentDirectional.topStart,

After positioning it as it should be this is the result 🌟✨✨

[HansMuller]

CC @xu-baolin

[github-actions]

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