Add GetMessages and DeleteMessage to MessageHandler

まず MessageHandler インターフェースと Message 構造体を見つけ、次にリポジトリ全体でそれらの実装と API の使用箇所を追跡します。メッセージ ID をどのように表現すべきか、また plugin に対する一覧表示、取得、削除がどのように動作すべきかを確認します。インターフェースと実装が要求された操作を一貫した動作でサポートすれば完了です。

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