[功能] 支持通过 PushPlus（推送加）渠道发送警报通知

Feature Request I would like to add PushPlus (https://www.pushplus.plus) as an alert notification channel to HertzBeat. PushPlus sends messages to channels such as WeChat via token + HTTP API, which is commonly used by individuals and small teams for monitoring and alerting. I have searched through documentation, existing issues/PRs, and code, but have not found any proposals or implementations related to PushPlus. The project already has third-party push notifications such as Server酱, Ntfy, and Gotify, but not PushPlus. If the community approves this direction, I am willing to take responsibility for development and submit a PR. Before starting to write code, I would like to ask the maintainer to confirm whether they accept making an independent notification channel.