[Feature]: Requesting Whatsapp support

## Summary

Requesting Whatsapp support. Whatsapp is by far the most popular messaging app in the world. In many countries, it's the primary chat app for 99% of people. https://www.statista.com/statistics/258749/most-popular-global-mobile-messenger-apps/ 3 billion monthly active users, more than double the amount of WeChat.

If CoPaw had Whatsapp support, I would be able to easily give chatbot access to CoPaw to my family and friends.

Thank you.

## Component(s) Affected

- [ ] Core / Backend (app, agents, config, providers, utils, local_models)
- [ ] Console (frontend web UI)
- [x] Channels (DingTalk, Feishu, QQ, Discord, iMessage, etc.)
- [ ] Skills
- [ ] CLI
- [ ] Documentation (website)
- [ ] Tests
- [ ] CI/CD
- [ ] Scripts / Deploy

## Problem / Motivation

Solves the most important missing channel in CoPaw, and the only reason I cannot use it.

## Proposed Solution

Add Whatsapp channel.

## Alternatives Considered

Installing other chat apps is an option: it works for me, but it won't work for my friends and family. I can't ask them to install a weird app like Telegram or Discord, which no one uses in our country.

## Additional Context

## Willing to Contribute

- [ ] I am willing to open a PR for this feature (after discussion).