Add GetMessages and DeleteMessage to MessageHandler #11
Description
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