Feature request: allow setting storage values while initializing app

# Feature request: allow setting storage values while initializing app

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

I have a use-case where I need to save a value when starting a new game. However, `eframe` provides `&eframe::CreationContext` and only allows access to `&dyn Storage`, which means values can be loaded from storage but cannot be set.

**Describe the solution you'd like**

Replace `storage: Option<&dyn Storage>` with `storage: Option<&mut dyn Storage>` in `CreationContext`.

**Describe alternatives you've considered**

Currently, I get around the aforementioned limitation by adding a `start_time_to_save: Option` field to my state type and running

```rust
if let Some(start_time) = self.start_time_to_save.take() {
eframe::set_value(frame.storage_mut().unwrap(), START_TIME_KEY, &start_time);
}
```

every `update`.

Start at eframe::CreationContext and trace how its storage reference is provided during app initialization. Review the existing frame.storage_mut() path and verify native and web initialization behavior; done means values can be written during creation without breaking existing storage reads or callers.

Written by the indexing model from the issue text.