Add metadata support to Message object in flutter_aepmessaging

### Prerequisites

- [x] This is not a Security Disclosure, otherwise please follow the guidelines in [Security Policy](https://github.com/adobe/aepsdk_flutter/security/policy).
- [x] I have searched in this repository's issues to see if it has already been reported.
- [x] I have updated to the latest released version of the SDK and the issue still persists.

### Bug summary

The Flutter flutter_aepmessaging plugin does not expose message metadata to Dart code, even though the native iOS and Android AEPMessaging SDKs support it.

Current Behavior
Message object only exposes id and autoTrack properties
Native bridges don't pass metadata from native SDK to Flutter
Metadata is always null/undefined in Dart code

Expected Behavior
Message object should include a metadata property of type Map?
Native bridges should pass metadata from native Message objects to Flutter
Developers can access metadata in MessagingDelegate callbacks (e.g., shouldShowMessage)

### Environment

Flutter flutter_aepmessaging v5.0.1

### Steps to reproduce

_No response_

### Current behavior

_No response_

### Expected behavior

_No response_

### Anything else?

_No response_

Start by tracing the Flutter Message object and the iOS and Android native bridges used by flutter_aepmessaging v5.0.1. Compare the native Message metadata with the values passed into MessagingDelegate callbacks such as shouldShowMessage; done means Dart can read the metadata as a nullable Map.

Written by the indexing model from the issue text.