Support for an file upload component (image upload, PDF upload etc)

It would be handy for some use cases to have a component that represents a file upload slot. Some thoughts:
- The contract could specify how the file is sent via A2A, which already has a FilePart etc
- The client would ideally show the uploaded file name and progress in the UI
- The UI would be flexible and look different on different devices e.g. a drop zone on web, or a button to open a picker on mobile
- There should be type filtering, to allow for specifically an image picker etc. Perhaps this is a usageHint.
- On mobile, it must be possible to make this an image picker which opens the phone gallery/camera