eframe: expose means of writing files to the file-storage directory location

[jleibs]

Is your feature request related to a problem? Please describe.
I am trying to store medium-sized binary files (not just serde) alongside the application state. I'd like to put them in the same directory where FileStorage is going to store other application state though.

Describe the solution you'd like
Option 1: Storage could expose a get_path API. This would be easier, though presents complications when running on web.

Option 2: Storage could expose an API like:

fn write_file(&mut self, key: &str, writer: FnOnce(impl std::io::Write));
fn read_file(&mut self, key: &str, reader: FnOnce(impl std::io::Read));

on web this probably needs to be backed by IndexedDB or OPFS.