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

**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.

Start by locating the FileStorage implementation and reviewing how its native file-storage directory is selected. Compare the proposed get_path and write_file/read_file APIs, including the stated web constraints around IndexedDB or OPFS; done means a decided, cross-platform storage interface.

Written by the indexing model from the issue text.