feat: add WeChat API integration

- Updated `Cargo.toml` to include `wechat-oa-sdk` dependency.
- Added WeChat configuration section in `config.toml` for app_id, app_secret, token, and optional encoding_aes_key.
- Implemented WeChat API routes in `mod.rs`, including user management, menu management, template messages, and customer service messages.
- Created `wechatapi.rs` to handle WeChat API logic, including message verification, user info retrieval, tag management, and QR code generation.
- Enhanced error handling and response structures for WeChat API interactions.

Author: ExquisiteCore