Cross Platform File Selection API

## Feature Request

Dioxus should have a cross platform api for programmatic file selection/saving.

Currently, we can use
```rust
rsx! {
input { r#type: "file" }
}
```
for file selection on web and desktop, and once https://github.com/DioxusLabs/dioxus/issues/3849 is resolved, mobile as well. Thus giving us a cross platform file selection element.

But there is no direct file selection api that can be invoked programmatically. On desktop we can use rfd directly with e.g. `rfd::FileDialog::new().set_directory(&path).pick_file();`, but this does not work for web or mobile. Thus there should be an api that can be used directly that covers web, desktop, and eventually mobile.

On web, the `set_directory` option should be ignored, since browsers are sandboxed so it will not have an effect.