[feat] FileInput ref should expose a reset prop to clear its value

[suzubara]

Does your feature request relate to a specific USWDS component?

The FileInput

What USWDS Version is this feature present in?

This is not related to a USWDS feature.

Is your feature request related to a problem? Please describe.

The FileInput exposes an inputRef prop in order to allow users access to the underlying input element. However, the file input element is unique in that its value cannot be set programmatically. This means that simply exposing a ref to the input element is not enough to allow a library user to reset the files selected by this input, which might be desirable if the user were implementing an async upload handler and wants to avoid rendering files in the FileInput component once they've been uploaded.

Describe the solution you'd like

We should be able to use React's ref forwarding alongside the useImperativeHandle hook to customize the ref that is returned by FileInput and expose a method to parent components that can be called to reset the file state to an empty array.

Additional context

We may want to start adopting this pattern for form control components that are more complex than a single input element (such as the ComboBox & DatePicker) and might also warrant exposing additional methods through a ref. This issue should help us get more understanding around this pattern, the use cases it fulfills, and how to move forward to ensure our form controls are highly adaptable for a variety of use cases.

This issue depends on #900 and will likely result in a major version update.