feat(channel): add WhatsApp channel support

**Background**

The channel system in Agent Service currently supports Feishu and Discord. WhatsApp is one of the most widely used messaging platforms globally, and adding WhatsApp channel support will significantly broaden the reach of Agent Service in international and consumer-facing scenarios.

**Changes**

- Add a new `WhatsAppChannel` class under `agentscope.app.channel` module (inherited from `ChannelBase`), considering to support (depends on the WhatsApp Business API side):
- `ListChats`: list all available conversations accessible by the bot
- `ListChatMembers`: list all members in a specified conversation
- `SendMessage`: send a text message to a specified user or group
- `SendFile`: send a file attachment to a specified user or group
- `SendImage`: send an image to a specified user or group
- More tools to be considered based on the capabilities provided by the WhatsApp Business API

Inspect `agentscope.app.channel` and the existing Feishu/Discord channel implementations to infer the expected `ChannelBase` contract. Add a new `WhatsAppChannel` class under this module implementing `ListChats`, `ListChatMembers`, `SendMessage`, `SendFile`, and `SendImage` based on WhatsApp Business API capabilities. Compare it with the repository’s channel integration pattern, and define done as a class that follows the same shape and wiring style while exposing the requested methods.

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