add fileupload component

We should probably take some opinion on how file uploading works with a2ui, need to explain how files are encoded with action button

low pri

Look for existing UI components in the a2ui codebase to understand the component structure. Investigate how file uploading is currently handled or if there are similar input components. The issue mentions explaining file encoding with an action button, so check documentation or examples for action buttons. Determine what a file upload component should look like and how it integrates with the existing design system.

索引モデルが issue の本文から書いたものです。