feat: implement optional group property for Preferences API

## Feature Request

### Plugin
@capacitor/preferences

### Description
Currently, all preferences share a global flat namespace configured globally by the `configure({group})` API.

This makes it difficult to manage separate logical buckets (e.g., App Settings vs. User Preferences) or clear specific sets of data without affecting others, especially during app initialization.

### Platform(s)
Android, iOS, Web

### Preferred Solution
I'd like to add an optional `group: string` property in `GetOptions`, `SetOptions`, `RemoveOptions`.
Creating an optional argument `KeysOptions` with `group: string`.

This property would be set to the current configured group by default.