feat: add asynchronous support to core notifier engines and HTTP providers

[Harshidpatel12]

Summary

This PR introduces native asynchronous support to the notifiers library, allowing notifications to be sent and resources to be queried asynchronously without blocking the event loop in ASGI/async frameworks (like FastAPI or Sanic).

Core Features Added

1. Core Async Interface:
   • Added notify_async and fallback handlers to the base Provider and ProviderResource classes, allowing all existing providers to run asynchronously inside the thread pool executor by default.
   • Added a top-level package helper notify_async (exposed in __init__.py).
2. Dynamic Async Request Wrapper:
   • Implemented AsyncRequestsHelper in notifiers/utils/requests.py using a dynamically imported httpx client to avoid adding heavy mandatory dependencies.
3. Optional Package Extras:
   • Added async optional dependencies extra to pyproject.toml (pip install notifiers[async]).
4. Native Async Support for HTTP Providers:
   • Migrated all HTTP-based providers (Slack, Telegram, Gitter, Mailgun, Pushover, Twilio, Zulip, PagerDuty, Pushbullet, DingTalk, VictorOps, PopcornNotify, SimplePush, Notify, Statuspage, and Join) to use native async HTTP calls.
5. Detailed Documentation:
   • Updated README.md with installation details and async code examples.
6. Tests:
   • Created a comprehensive async test suite in tests/test_async.py verifying both thread-executor fallback logic and mocked native async HTTP calls.