Add GetMessages and DeleteMessage to MessageHandler

首先定位 MessageHandler 接口和 Message 结构体，然后追踪它们在整个仓库中的实现和 API 使用情况。确定消息 ID 应如何表示，以及对于 plugin，列出、获取和删除应如何工作。接口和实现支持所请求的操作并具有一致的行为，即表示完成。

Currently, the MessageHandler has an API gap in that it cannot list, get, or delete messages. This would be valuable for plugins who want to display a persistent notification while some event is ongoing.

I think the MessageHandler interface could just be extended with a few extra functions

// MessageHandler consists of message callbacks to be used by plugins.
type MessageHandler interface {
	// SendMessage sends a message with the given information in the request.
	SendMessage(msg Message) error

	// GetMessages returns all active messages sent by this plugin.
	GetMessages() ([]Message, error)

	// GetMessage returns a specific message by ID.
	GetMessage(id uint) (*Message, error)

	// DeleteMessages deletes all messages created by this plugin.
	DeleteMessages() error

	// DeleteMessage deletes a specific message by ID.
	DeleteMessage(id uint) error
}

I think the Message struct might also need to be extended or a new struct would need to be defined that includes an ID.

As background, I want to make a plugin for Alertmanager where alerts will be persistently created as notifications in Gotify so that I only get one notification for the alert and it gets deleted automatically when it resolves. Additionally, I want to add special handling for the watchdog alert so if Alertmanager stops sending it, I get a notification. With the current plugin API, I am only able to send messages when I get them from alertmanager.

If this is a desirable inclusion, I would love to implement it.

Relates to #7