Add flutter_aepoptimize plugin for Adobe Optimize SDK

[akhiljain1907]

Description
Adds the flutter_aepoptimize plugin — a complete Flutter wrapper for the native AEP Optimize iOS and Android SDKs.

APIs:

Optimize.updatePropositions() / getPropositions() — fetch and retrieve personalization offers
Optimize.onPropositionsUpdate() — persistent listener for cache updates
Optimize.clearCachedPropositions() — clear client-side cache
Optimize.displayed() / generateDisplayInteractionXdm() — batch offer tracking
Offer.displayed() / tapped() — single offer tracking
Offer.generateDisplayInteractionXdm() / generateTapInteractionXdm() — XDM generation
OptimizeProposition.generateReferenceXdm() — proposition-level XDM
Models: DecisionScope, OptimizeProposition, Offer, OfferType

Multiple native API overloads (no-callback, callback, callback+timeout) are consolidated into single Dart methods with optional parameters.

Motivation and Context
The AEP Optimize SDK was the only remaining AEP extension without a Flutter plugin. This brings Flutter to parity with the existing React Native Optimize plugin, enabling Flutter apps to use Adobe Target and Offer Decisioning through the Edge Network.

How Has This Been Tested?
636 lines of Dart unit tests covering:
API method invocation and argument serialization
Response decoding from method channel
Model roundtrips (fromMap / toMap)
Enum conversions (int ↔ OfferType, MIME string mapping)
Listener callback registration and firing
Edge cases (null/missing fields, empty responses)
Manual testing on iOS simulator and Android emulator via example app

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.