Expose public API to open native attribution dialog programmatically (for custom attribution buttons)

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I am using `mapbox_maps_flutter` and need to support a custom in-app attribution button (larger hit target / custom UI layout), while still opening the **native Mapbox attribution dialog** (including telemetry controls where applicable).

At the moment, there does not seem to be a reliable/public Flutter API for this workflow. I can configure attribution ornament visibility/position, but cannot explicitly trigger the native dialog from a custom button.

## Why this matters

- Apps frequently need custom controls for accessibility and usability (larger touch areas, custom placement, integrated control panels).
- Attribution still needs to be available for compliance.
- We also need to preserve native attribution behavior and telemetry settings

## Current behavior

- `mapbox_maps_flutter` exposes attribution settings, but no documented method like:
- `mapboxMap.showAttributionDialog()`
- `mapboxMap.openAttribution()`
- Bridging from host app native code is fragile and version-dependent (ornament internals are not a stable public surface).

Please expose a public, stable API in `mapbox_maps_flutter` to open the native attribution flow programmatically from app code, for example:
```dart
await mapboxMap.showAttributionDialog();
```

Start by reviewing the existing attribution ornament settings and the platform-specific native attribution flow in this Flutter plugin. Trace how Dart APIs reach the native Mapbox SDKs, then define and document a public method whose completion criteria include opening the native dialog and preserving applicable telemetry controls on supported platforms.

Written by the indexing model from the issue text.