v0.0.1 of instant_request_handler

Reusable Flutter plugin for immediate incoming request notification experiences. Changelog excerpt: - Initial package scaffold for generic incoming request notification handling. - Added Dart registration API, request state, timeout flow, duplicate protection, pending request recovery, and default UI widgets. - Added Android Kotlin bridge, FCM service, full-screen/head-up notification, wake lock, permission helpers, and native pending storage. - Added example app and setup documentation.